Reference

Last reviewed: 28 Apr 2026
Sources cited: 10

Compound class~4,114 Da

Modified GLP-1 receptor agonist

31-residue analogue of human GLP-1 (94% sequence homology) with C18 diacid conjugate enabling ~1-week plasma half-life via albumin binding.

Semaglutide

Last verified: 2026-04-24

At a glance


Also known as	Ozempic (brand — type 2 diabetes, injectable), Wegovy (brand — obesity/cardiovascular, injectable), Rybelsus (brand — oral tablets for T2DM), NN9535
Class	GLP-1 receptor agonist (single mechanism, unlike tirzepatide’s dual GLP-1/GIP)
Typical route	Subcutaneous injection weekly (Ozempic/Wegovy); oral tablet daily (Rybelsus)
UK status	MHRA-approved prescription-only medicine (POM). Available through NHS (restricted) and private prescription.
Plasma half-life	~1 week (very long; enables once-weekly dosing for injectables)
Oral bioavailability (Rybelsus)	~0.4–1% — extremely low, requires co-formulation with absorption enhancer
Molecular weight	~4,114 Da (31 amino acids; modified analogue of human GLP-1 with 94% sequence homology)

What it is

Semaglutide is a licensed pharmaceutical medicine that predates tirzepatide by several years and has significantly more real-world use and long-term safety data. Three formulations exist, all containing the same active molecule:

Ozempic — once-weekly injection, licensed for type 2 diabetes. Major additional approval for cardiovascular risk reduction in T2DM patients.
Wegovy — once-weekly injection at higher doses (2.4 mg maintenance), licensed for chronic weight management in obesity. Also FDA-approved for cardiovascular risk reduction in obese patients without diabetes.
Rybelsus — once-daily oral tablet, licensed for type 2 diabetes. The first oral GLP-1 receptor agonist.

Unlike tirzepatide’s dual mechanism, semaglutide is a pure GLP-1 receptor agonist — activating only GLP-1, not GIP. The single mechanism is simpler, has more historical evidence, and produces slightly less weight loss than tirzepatide per unit effort — but comes with one major evidence advantage: SELECT (2024), a 17,604-patient trial showing semaglutide reduces major adverse cardiovascular events by 20% in obese/overweight patients with pre-existing cardiovascular disease, independent of diabetes status.

For UK users, semaglutide is typically:

Cheaper than tirzepatide privately (though not always — depends on dose and provider)
More available through NHS than tirzepatide (Wegovy has broader Tier 3 access)
Better-evidenced for cardiovascular benefit (SELECT is the largest CV outcomes trial in obesity)
Less weight-loss effective than tirzepatide (15–17% vs 21% at maximum doses)

See tirzepatide for direct comparison — users genuinely choose between the two, and the trade-offs matter.

Mechanism

Semaglutide activates the GLP-1 receptor, producing downstream effects consistent with enhanced endogenous GLP-1 signalling:

Appetite reduction. Central GLP-1 receptors in the hypothalamus modulate satiety signalling; activation reduces hunger and enhances satiety.
Delayed gastric emptying. Food spends longer in the stomach; meals feel more filling for longer.
Enhanced glucose-dependent insulin secretion. Pancreatic β-cells release more insulin when blood glucose is elevated (but not when it’s normal — hence low hypoglycaemia risk compared to insulin or sulphonylureas).
Suppressed glucagon release. Reduces hepatic glucose production; part of the glucose-lowering effect.
Cardiovascular benefit independent of weight. The SELECT trial demonstrated cardiovascular benefit that was only modestly associated with waist circumference reduction, suggesting mechanisms beyond adiposity reduction (anti-inflammatory effects, endothelial function, possibly direct cardiovascular effects).
Albumin binding for long half-life. Semaglutide’s ~1-week plasma half-life results from its binding to plasma albumin (94% of human GLP-1 sequence with specific modifications to enable this binding). This protection from metabolic degradation and renal clearance is what enables once-weekly dosing.

Compared to tirzepatide: tirzepatide adds GIP receptor activation on top of GLP-1. The GIP component contributes to additional weight loss and improved metabolic effects, which is why tirzepatide outperforms semaglutide head-to-head in weight loss — but not by enough to make semaglutide obsolete, especially given semaglutide’s stronger cardiovascular data and longer safety track record.

Routes of administration

Subcutaneous injection — Ozempic (type 2 diabetes)


Form	Prefilled injection pens at 0.25 mg, 0.5 mg, 1 mg, 2 mg
Frequency	Once weekly, same day each week
Starting dose	0.25 mg for 4 weeks (tolerance-building, not therapeutic)
Maintenance dose	0.5–2 mg depending on response
Maximum licensed dose	2 mg weekly (for T2DM)
Equipment	Pen is self-contained — no reconstitution

The T2DM formulation. Lower maintenance doses than Wegovy. Fundamentally the same active molecule, different licensed dose ceiling.

Subcutaneous injection — Wegovy (obesity / cardiovascular risk)


Form	Prefilled injection pens at 0.25 mg, 0.5 mg, 1 mg, 1.7 mg, 2.4 mg, and (new for UK 2026) 7.2 mg
Frequency	Once weekly, same day each week
Starting dose	0.25 mg for 4 weeks
Standard maintenance	2.4 mg weekly
Higher dose	7.2 mg — approved UK 12 January 2026 for patients plateaued on 2.4 mg with minimal side effects
Equipment	Pen is self-contained

The weight-management formulation. Higher dose ceiling than Ozempic. The January 2026 UK approval of 7.2 mg is notable — addresses the “Wegovy plateau” many users experienced after 12–18 months at 2.4 mg.

Oral — Rybelsus (type 2 diabetes)


Form	Tablets at 3 mg, 7 mg, 14 mg (UK new formulation includes 25 mg as of 2025)
Frequency	Once daily, same time each day
Bioavailability	~0.4–1% — extremely low (characteristic of peptide oral delivery)
Absorption conditions	Strict — must be taken on empty stomach with ≤120 mL water, then fast for 30 minutes before eating/drinking/other medications
When this route makes sense	Type 2 diabetes patients preferring oral dosing; alternative for needle-averse users; absorption conditions must be manageable

The dosing conditions are genuinely strict and not optional. Taking Rybelsus with food, with too much water, or not fasting afterward dramatically reduces absorption. This is why most clinicians recommend injectable semaglutide unless oral is specifically preferred.

Oral semaglutide is co-formulated with salcaprozate sodium (SNAC), an absorption enhancer that temporarily opens tight junctions in the stomach epithelium to allow the peptide through. Without SNAC, oral semaglutide would have essentially zero bioavailability.

Rybelsus is not approved for weight management. A separate oral semaglutide for obesity indication was approved in the US in 2025 but is not yet widely available; UK approval for oral weight-management semaglutide was pending as of April 2026.

Compounded / grey-market semaglutide

Covered under “Areas of concern.” This is not a licensed access route.

Cross-route comparison

Scenario	Best option
Type 2 diabetes, weekly dosing preferred	Ozempic
Obesity without diabetes	Wegovy
Diabetes + established cardiovascular disease	Ozempic (CV indication)
Obesity + established cardiovascular disease	Wegovy (2024 CV indication)
Type 2 diabetes, needle-averse, strict dosing manageable	Rybelsus
Weight loss, needle-averse	Wait for licensed oral weight-management semaglutide OR accept Wegovy injection

Injectable semaglutide is more effective than oral due to predictable bioavailability. Oral is convenient but the absorption conditions are a real ongoing burden.

What the evidence says

Honest summary: semaglutide has the largest and most mature evidence base of any compound in this encyclopedia. Multiple Phase 3 programmes (STEP, SUSTAIN, SELECT) totalling tens of thousands of participants. Cardiovascular outcomes trial (SELECT) is particularly important and lacks a tirzepatide equivalent as of 2026.

STEP programme (weight loss — the primary evidence for Wegovy)

Four main trials (STEP 1, 3, 4, 8) evaluating semaglutide 2.4 mg weekly for 68 weeks:

Mean weight loss: 14.9–17.4% (vs ~3% on placebo)
69–79% of participants achieved ≥10% weight loss
51–64% achieved ≥15% weight loss
Generally well tolerated; ~10% discontinuation for GI events
Consistent cardiometabolic improvements (blood pressure, lipids, glucose)1

SUSTAIN programme (type 2 diabetes — the primary evidence for Ozempic)

Seven main trials across the T2DM spectrum, >8,000 patients:

HbA1c reductions of 1.0–1.8% depending on dose and comparator
Superior to many other diabetes treatments in head-to-head
SUSTAIN-6 demonstrated significant reduction in cardiovascular events in high-CV-risk T2DM patients — the first cardiovascular outcomes trial for semaglutide2

SELECT (the flagship cardiovascular outcomes trial)

17,604 adults with pre-existing cardiovascular disease, overweight or obesity, WITHOUT diabetes. This trial matters because it shows cardiovascular benefit in the population most commonly taking semaglutide for weight loss.

Primary endpoint: composite of cardiovascular death, nonfatal MI, nonfatal stroke
Result: 20% reduction in MACE with semaglutide 2.4 mg weekly vs placebo
Weight loss in the trial: mean 9.4% — meaningful but not the dramatic STEP programme numbers
Critical finding: cardiovascular benefit was only modestly associated with weight loss magnitude, suggesting direct cardioprotective mechanisms beyond adiposity reduction3
Led to FDA-approved indication for cardiovascular risk reduction in obesity without diabetes (2024) — first ever for any medication in this class

This is why cardiologists and some weight-management specialists prefer semaglutide over tirzepatide for patients with established cardiovascular disease. Tirzepatide’s equivalent trial (SURPASS-CVOT) is ongoing as of 2026 but has not yet read out.

Head-to-head with tirzepatide (SURMOUNT-5)

Tirzepatide outperformed semaglutide for weight loss — 20.2% vs 13.7% mean weight loss.4 For patients prioritising weight loss, tirzepatide has the edge. For patients prioritising cardiovascular benefit with established CV disease, semaglutide has the better evidence as of 2026.

Real-world evidence

Semaglutide has been in widespread clinical use since 2017 (Ozempic) and 2021 (Wegovy). Hundreds of millions of patient-months of cumulative use. The long-term safety picture is consequently clearer than for tirzepatide.

Typical use patterns

Unlike research chemicals, semaglutide dosing comes from licensed prescribing information.

Wegovy titration (weight management)

Weeks	Dose
1–4	0.25 mg weekly (tolerance-building)
5–8	0.5 mg weekly
9–12	1.0 mg weekly
13–16	1.7 mg weekly
17+	2.4 mg weekly (standard maintenance)
Plateau with tolerance	7.2 mg weekly (UK available January 2026)

Ozempic titration (type 2 diabetes)

Weeks	Dose
1–4	0.25 mg weekly
5+	0.5 mg weekly (first maintenance)
After 4+ weeks	1.0 mg weekly if needed
After 4+ weeks	2.0 mg weekly (maximum for T2DM)

Rybelsus titration (oral T2DM)

Duration	Dose
First 30 days	3 mg daily (not therapeutic — tolerance/absorption building)
Thereafter	7 mg daily (first maintenance)
If needed	14 mg daily
Newer	25 mg daily (UK availability varies)

Timing and practical considerations

Injectable semaglutide: same day each week. Can be shifted ±4 days without missing dose.
Rybelsus: same time each day, strict fasting protocol (empty stomach, ≤120 mL water, 30 min fast after)
Injection sites: abdomen, thigh, or upper arm. Rotate.
Timing of other oral medications: Rybelsus’s fasting window means other oral meds need careful timing

Duration of treatment

Like tirzepatide, semaglutide is chronic treatment for its licensed indications. Discontinuation typically results in weight regain for obesity users and deterioration of glucose control for diabetes users. This is not a short-term intervention.

Long-term users on semaglutide for 3+ years are now common; the drug has been in use long enough to establish real-world long-term patterns.

For sensitive systems

Semaglutide shares most of tirzepatide’s sensitive-systems considerations (see tirzepatide “For sensitive systems”). Key differences:

Nausea profile:

Semaglutide reportedly produces more frequent nausea/vomiting than other GLP-1 agonists in the class — including potentially more than tirzepatide despite tirzepatide having similar GLP-1 effects
However individual tolerability varies: some users find semaglutide easier than tirzepatide, others the reverse

Slower dose escalation option:

Starting dose (0.25 mg for Wegovy) is already lower in absolute mg terms than tirzepatide’s 2.5 mg
But the therapeutic ceiling is also lower proportionally
Extended titration (longer than 4 weeks per step) is common in clinical practice for sensitive users

For MCAS / histamine-sensitive users:

Same considerations as tirzepatide — slowed gastric emptying may aggravate histamine/gut issues
No specific mast cell provocation signal
Injection site reactions typically mild

For POTS / autonomic dysfunction users:

Dehydration risk from nausea/vomiting is real
Start low, titrate slowly, maintain hydration and electrolytes
Resting heart rate may increase by 2–3 bpm on semaglutide — not usually clinically significant but worth knowing for POTS users whose baseline is already elevated

For ME/CFS / chronic fatigue:

Early-weeks fatigue is common
Slow titration essential; some users need 6–8 weeks at each step rather than 4
Rybelsus (oral) may be even harder to tolerate due to the rigid fasting protocol

For gastroparesis:

Semaglutide slows gastric emptying — additive to existing gastroparesis
Relative contraindication depending on severity

For PMDD:

Weight and metabolic stability can benefit cyclical symptoms in some users
Nausea can be worse in luteal phase; dose timing may need adjustment across cycle

Interactions worth considering:

Insulin and sulphonylureas: increased hypoglycaemia risk. Requires medication dose adjustment.
Tirzepatide: do not combine — same class, additive risk, no clinical benefit
Liraglutide (Saxenda, Victoza): do not combine — same class
Oral medications: slowed gastric emptying affects absorption. For Rybelsus specifically, other oral medications need careful timing around the 30-minute fasting window.
Oral contraceptives: no specific interaction for injectable semaglutide; Rybelsus may theoretically affect absorption of other oral drugs but clinical impact on contraception is minimal
Warfarin: INR monitoring may need adjustment
Research-chemical peptides: no documented interactions; combining prescription medicine with research chemicals is a clinician-discussion matter
LDN, SSRIs/SNRIs: no documented interactions

For users comparing semaglutide vs tirzepatide:

Tirzepatide generally produces more weight loss and may have slightly better tolerance in some users
Semaglutide has stronger cardiovascular evidence as of 2026
Semaglutide has longer real-world safety history
Semaglutide is typically less expensive privately, but not always
Individual response varies — some users tolerate one but not the other
Starting with semaglutide and switching to tirzepatide if inadequate response is a reasonable sequence; reverse is also reasonable

Reasonable expectations

Onset. Appetite suppression within days of first dose. Measurable weight loss from weeks 4–8. Significant weight loss by weeks 20–40. Full effect around week 68 in the STEP programme. Cardiovascular benefit accrues over years.

Response rate.

Weight management (Wegovy): ~69–79% achieve ≥10% weight loss at 68 weeks; ~51–64% achieve ≥15%. Significantly better than placebo but modestly less effective than tirzepatide.
Type 2 diabetes (Ozempic): most patients achieve meaningful HbA1c improvement; many achieve target HbA1c <7% without additional medication
Cardiovascular benefit (Wegovy/SELECT population): 20% relative risk reduction in MACE over ~3.3 years

What the evidence actually supports.

Weight loss in obesity: 15–17% average at therapeutic doses over ~16 months — well-established
Glycaemic control in T2DM: HbA1c reductions 1.0–1.8% — well-established
Cardiovascular risk reduction in T2DM (SUSTAIN-6): established
Cardiovascular risk reduction in obesity without diabetes (SELECT): established 2024
Prediabetes reversal: documented, though effect size smaller than tirzepatide
Real-world long-term safety (3+ years): established, reassuring

What not to expect.

Tirzepatide-level weight loss. Head-to-head, tirzepatide wins on weight loss magnitude.
Permanent weight loss from a short course. Like tirzepatide, this is chronic treatment.
Benefits without continued use. Discontinuation → regain.
Effortless body composition improvement. Muscle loss with rapid weight loss is real; protein intake and resistance exercise remain essential.
Oral formulation matching injection effectiveness. Rybelsus is real but less effective than injectable.
Cardiovascular protection in patients without established CV disease or multiple CV risk factors. SELECT was in secondary prevention population; primary prevention benefit is plausible but unproven.

Cost

Wegovy private prescription

Dose / provider	Typical monthly cost
0.25 mg (starter)	£130–200
0.5 mg	£150–220
1.0 mg	£170–250
1.7 mg	£190–270
2.4 mg (standard maintenance)	£200–280
7.2 mg (newer 2026 option)	pricing still settling; expect premium

Ozempic private prescription

Typically cheaper than Wegovy due to lower licensed dose ceilings:

Dose	Typical monthly cost
0.25 mg	£140–180
0.5 mg	£150–200
1 mg	£160–230
2 mg	£180–250

Some private providers will dispense Ozempic off-label for weight loss at higher doses; this sits in a grey area and is less common post-2023 as Wegovy availability expanded.

Rybelsus private prescription

Dose	Typical monthly cost
3 mg (starter)	£150–200
7 mg	£170–220
14 mg	£180–250

NHS access

Wegovy via Tier 3 specialist services: eligibility typically BMI ≥35 (or 32.5 for certain ethnic groups) with comorbidity. NHS prescription charge £9.90 (England); free in Scotland/Wales/NI.
Ozempic on NHS: available for type 2 diabetes; standard prescription charges
Rybelsus on NHS: available for type 2 diabetes; standard prescription charges
Wegovy NHS availability is better than tirzepatide NHS availability — historically more prescribing pathways exist

UK provider comparison

Same GPhC-registered providers as tirzepatide: Superdrug, Boots, Asda, Numan, Voy, LloydsPharmacy, Medino, Bolt, The Family Chemist, My London Pharmacy.

Provider choice criteria (same as tirzepatide):

GPhC registration
MHRA authorisation
Proper clinical review
Clear sourcing of licensed medication
Ongoing support

Cost comparison with tirzepatide

At equivalent efficacy doses: semaglutide is generally cheaper by £30–80/month
At maximum licensed doses: the gap narrows or inverts (Wegovy 7.2 mg is competitive with tirzepatide 15 mg)
Ozempic vs Mounjaro for diabetes: Ozempic typically cheaper
For users optimising cost per unit weight loss: tirzepatide may actually be more cost-effective despite higher per-month cost, because it produces more weight loss

Reconstitution

This section covers reconstitution of research-chemical semaglutide. Wegovy, Ozempic, and Rybelsus arrive as prefilled pens or oral tablets and require no preparation — if that’s your route, skip this section. Semaglutide is the most widely-licensed of the three GLP-1 compounds in this encyclopedia, which means more users genuinely have a pen-or-tablet pathway available; reconstitution is for the cohort buying lyophilised powder from research-chem vendors, typically because of cost, supply gaps, or specific dosing patterns the licensed pens can’t deliver.

What’s in the box

Research-chemical semaglutide ships as lyophilised white powder in a glass vial, typically 5 mg or 10 mg per vial. 3 mg and 15 mg vials exist but are less commonly stocked. The vial is sealed with a rubber stopper and aluminium crimp; you draw through the stopper.

You’ll also need:

Bacteriostatic water (BAC water) — sterile water with 0.9% benzyl alcohol. The 28-day stability window for semaglutide depends on the benzyl alcohol; plain saline or sterile water doesn’t preserve it.
Insulin syringe (U-100) — typically 1 mL with 29G–31G needle. 100 units = 1 mL, so each unit = 0.01 mL.
A larger syringe (3 mL or 5 mL) for transferring the BAC water into the vial. Insulin syringes draw too slowly through stoppers.
Alcohol wipes, a clean working surface, and somewhere to record what you did.

The three practitioner camps

Semaglutide users split into three patterns. Compared to retatrutide, the picture shifts because semaglutide has been licensed since 2017 — the licensed pen schedule sets the baseline that the other two camps deviate from.

The standard titration camp follows the Wegovy schedule: 0.25 → 0.5 → 1.0 → 1.7 → 2.4 mg/week, escalating every 4 weeks. One injection per week. This is the camp the trial data and the licensed prescribing schedule were designed for. Research-chem users typically buy 5 mg or 10 mg vials and follow the same dose ladder.

The split-dosing camp takes the same total weekly dose and divides it into two injections, typically Monday and Thursday — for example, 1 mg/week becomes 0.5 mg twice-weekly. The community sometimes calls this “microdosing,” which is a misnomer: real microdosing means an order of magnitude below the therapeutic range, and split dosing keeps the total weekly dose unchanged. Only the per-injection peak is smaller.

Semaglutide is unusually well-suited to split dosing because its plasma half-life is approximately seven days, the same as the standard dose interval. By the time the next weekly injection lands, ~50% of the previous dose is still circulating. Splitting twice weekly produces noticeably flatter peak-to-trough swings; pharmacokinetic modelling suggests ~20–25% lower peaks while preserving 80–95% of efficacy. Beyond twice weekly, the long half-life means three-times-weekly dosing offers minimal additional benefit.

The sustained-low-dose camp runs absolute weekly doses below the standard titration floor — typically 0.125–0.25 mg/week — held flat over a cycle rather than escalated. The goals here are usually metabolic rather than weight-loss-focused: improved insulin sensitivity, modest appetite control, cardiometabolic markers, lower per-month cost. Ozempic at 0.25 mg sustained is the most common starting point. This is also sometimes called “microdosing” in the community, but it’s the low end of normal semaglutide dosing rather than a true microdose.

All three patterns are real and used widely. None is “wrong” — they’re different goals. The reconstitution math just needs to match the goal.

The math

For research-chem reconstitution, the rule is: pick a BAC volume that puts each injection at a syringe mark you can read accurately. U-100 insulin syringe markings get hard to read below ~5 units (0.05 mL), so aim higher than that for accuracy. Semaglutide’s per-injection doses are about an order of magnitude smaller than retatrutide’s, so dilution choice matters more for readability.

Standard titration — 5 mg vial, 2 mL BAC water (the most commonly cited setup)

Concentration: 2.5 mg/mL (2,500 mcg/mL)
0.25 mg weekly = 0.1 mL = 10 units
0.5 mg weekly = 0.2 mL = 20 units
1.0 mg weekly = 0.4 mL = 40 units
1.7 mg weekly = 0.68 mL = 68 units
2.4 mg weekly = 0.96 mL = 96 units (right at the readable upper edge of a 1 mL syringe)

Standard titration — 10 mg vial, 2 mL BAC water

Concentration: 5 mg/mL
0.25 mg = 0.05 mL = 5 units (the readability floor)
0.5 mg = 0.1 mL = 10 units
1.0 mg = 0.2 mL = 20 units
2.4 mg = 0.48 mL = 48 units

Split dosing — 5 mg vial, 2 mL BAC water

Same 2.5 mg/mL concentration; the math is per-injection.
0.5 mg/week as 0.25 mg twice weekly = 0.1 mL = 10 units, twice weekly
1.0 mg/week as 0.5 mg twice weekly = 0.2 mL = 20 units, twice weekly
2.4 mg/week as 1.2 mg twice weekly = 0.48 mL = 48 units, twice weekly

Sustained low dose — 5 mg vial, 2.5 mL BAC water

Concentration: 2 mg/mL (2,000 mcg/mL)
0.125 mg (125 mcg) weekly = 0.0625 mL = 6 units (right at the readability floor; consider slightly less BAC for a friendlier number)
0.25 mg weekly = 0.125 mL = 12.5 units (round to 12 or 13 in practice)

The Peptrax Vial Plan calculator handles the same math interactively — enter your vial size, BAC volume, and weekly dose, and it returns draw volume, doses per vial, and depletion timing.

Reconstitution procedure

Wipe the rubber stopper of the lyophilised vial and the BAC water vial with alcohol.
Draw the chosen volume of BAC water into the larger syringe.
Insert the needle into the lyophilised vial at an angle, with the tip touching the inside wall of the vial above the powder. Inject the water slowly down the inside of the vial, not directly onto the powder. Forcing water onto lyophilised peptide can denature it.
Leave the vial undisturbed for 1–2 minutes. Do not shake. Gently swirl if needed; the powder will dissolve on its own. Shaking creates foam and can fragment peptide chains.
Once the solution is clear, label the vial: compound, concentration, reconstitution date.
Store refrigerated (2–8°C / 36–46°F).

Some practitioners pass the reconstituted solution through a 0.22 µm syringe filter as a belt-and-braces sterility step. With a sealed lyophilised vial and clean technique, this is optional rather than standard — the vial was sterile when sealed, and your BAC water contains benzyl alcohol. Filtering is more relevant if you’re concerned about a specific vendor’s lyophilisation quality or if you’re using plain (non-bacteriostatic) sterile water for a single-use prep.

Storage and stability

Semaglutide reconstituted with BAC water is better-characterised than retatrutide because it has been on the market since 2017 and there’s more vendor and pharmacist guidance. The honest range:

Conservative: 14–21 days refrigerated. Some compounding pharmacies and research-supply vendors give this as the safe window.
Middle: 28 days refrigerated. The most commonly cited figure across community sources, peptide vendors, and compounded-semaglutide guidance. Tracks with Wegovy’s once-opened pen shelf life of 28 days at refrigerator temperatures.
Optimistic: ~30 days refrigerated. Within the same window; the rare claims of longer (45+ days) come from research-grade laboratory storage that doesn’t translate to grey-market reconstitutions.

The honest middle is 28 days refrigerated at 2–8°C. This is the figure the licensed Wegovy pen uses post-opening, and the reconstituted research-chem solution should perform similarly when properly handled.

Store at the back of the refrigerator where the temperature is most stable, not in the door. Do not freeze — ice crystal formation disrupts peptide structure and reduces potency on thaw.

If you reconstitute with plain sterile water instead of BAC water, the vial is single-use — discard within 24 hours because there’s no preservative. Some sources stretch this to 48 hours; conservative practice is 24.

The stability constraint matters across all three camps for semaglutide because doses are small. A 5 mg vial dosed at 0.25 mg/week is 20 weeks of theoretical doses but only ~4 weeks of stable solution — so the sustained-low-dose camp will discard 70–80% of any standard-size vial. The split-dosing camp at 0.5 mg twice-weekly (1 mg/week total) gets 5 weeks of theoretical doses from a 5 mg vial — just over the 28-day stability window, so plan for a small loss. Only the standard titration camp at 1.7–2.4 mg/week comfortably uses the whole vial within shelf life.

What gets miscalculated

The recurring grey-market reconstitution errors for semaglutide, in rough order of frequency:

Confusing mg, mcg, and units. 1 mg = 1,000 mcg. A “10-unit” dose on a U-100 syringe is 0.1 mL of solution, not 10 mcg of compound. Semaglutide’s small absolute doses make this easy to fumble: 0.25 mg ≠ 25 units.
Reading “units” as mg. “Take 10 units” is not “take 10 mg.” Units are a volume mark on the syringe.
Using Wegovy/Ozempic pen dose schedules with research-chem vials without doing the conversion. The pens deliver a fixed mg per click; the powder needs to be reconstituted to a chosen concentration first.
Underestimating shelf life loss at low doses. Sustained-low-dose users running 0.25 mg/week from a 5 mg vial discard most of the vial. Either accept the loss, find smaller vials (3 mg are sometimes available), or use BAC volume / dilution choices that deliberately match dose count to stability window.
Drawing dose by eye on insulin syringes below 5 units. Below ~5 units, drawing accuracy drops sharply. Semaglutide’s small doses regularly land at 5–10 units; always check your concentration choice doesn’t push the dose under that line.

Areas of concern ⚠

Grey-market and compounded semaglutide (the ongoing UK concern)

Compounded semaglutide is unlicensed medicine under UK law.

The situation is particularly complex for semaglutide:

Patent expiry in multiple countries (India, Brazil, China, Turkey) is fuelling grey market growth
In the UK, patent protection extends to 2031 and beyond — so domestic manufacture of generic semaglutide is not legal
Cross-border shipping of generic semaglutide from patent-expired countries is treated as unlicensed supply by MHRA
MHRA enforcement: In late 2025 and ongoing 2026, MHRA has shut down illicit manufacturing facilities (East Midlands raid yielding 2,000 unlicensed GLP-1 pens) and issued enforcement actions against direct and indirect GLP-1 product advertising
ASA enforcement: Advertising Standards Authority issued a September 2025 enforcement notice — any direct or indirect reference to GLP-1 products in public-facing promotions violates UK law

Why this matters beyond legality:

Cold chain integrity: illicit shipping rarely maintains the strict refrigeration required. Users may be injecting degraded, biologically inactive protein.
Purity and contamination: no MHRA oversight, no batch testing, no quality control
Dosing accuracy: underpotent or overpotent product documented in grey-market samples
No medical oversight: no screening for contraindications, no ongoing clinical support

If cost is a barrier, compare licensed UK providers directly before assuming grey-market supply is the only cheaper route. Some regulated providers offer bundled care or lower introductory pricing.

Gallbladder and biliary disease (increased risk)

Semaglutide-treated patients: 0.6% gallbladder events vs 0.1% placebo
Now included in safety labelling
Rapid weight loss contributes; not entirely a direct drug effect
Not a contraindication for users without pre-existing gallbladder disease, but monitor for RUQ pain

Pancreatitis

Meta-analyses have NOT found increased pancreatitis risk vs placebo
However, multiple case reports exist — the signal is weak at population level but real at individual level
Not a contraindication without history, but history of pancreatitis is a reason for specialist involvement
Acute severe abdominal pain during treatment → stop, seek immediate medical review

Thyroid cancer concern

Boxed warning based on rodent studies showing thyroid C-cell tumours
Human data remain inconclusive — limited cases observed, marginal GLP-1 receptor expression in human thyroid tissue
Contraindicated in personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia type 2 (MEN2)
Not demonstrated to cause thyroid cancer in humans as of 2026

Diabetic retinopathy (specific to T2DM users)

SUSTAIN-6 showed increased retinopathy complications
Mechanism: secondary to rapid glycaemic improvement, not direct drug effect
Relevant for T2DM patients with existing retinopathy at baseline — requires ophthalmology coordination
Not relevant for non-diabetic Wegovy users

Cardiovascular concern (actually good news here)

Modest heart rate increase (2.75–3.2 bpm) — no clinical cardiac events
Large trials (SELECT, SUSTAIN-6) demonstrate CV benefit, not risk
This is one of the reassuring aspects of semaglutide

Rapid weight loss complications (same as tirzepatide)

Gallstones (as above)
Muscle mass loss if insufficient protein and resistance exercise
Nutritional deficiencies
Volume loss / “Ozempic face”
Loose skin with significant weight loss

Long-term safety (better-characterised than tirzepatide)

Clinical use since 2017 (Ozempic), 2021 (Wegovy)
Real-world patient-months in the hundreds of millions
No signals of unexpected long-term toxicity at current scale
5-year and longer use is common in clinical practice

The dependency question

Same as tirzepatide: semaglutide works as long as you take it. Weight regain on discontinuation is documented and common. Frame as chronic disease management, not time-limited intervention.

Populations where semaglutide is contraindicated or requires caution

Personal or family history of medullary thyroid carcinoma (MTC) — contraindicated
Multiple Endocrine Neoplasia syndrome type 2 (MEN 2) — contraindicated
Pregnancy — contraindicated; effective contraception required, discontinue ≥2 months before planned conception
Severe gastroparesis — relative contraindication
History of pancreatitis — caution; specialist input
Proliferative diabetic retinopathy (T2DM users) — ophthalmology coordination required
Type 1 diabetes — not licensed for T1DM; specialist only if combined with insulin
Under 18 — licensed for adolescents aged 12+ for Wegovy only; not for other semaglutide formulations without specialist context
Severe renal or hepatic impairment — specialist review

FDA / regulatory status

Jurisdiction	Status	Last verified
UK (MHRA)	MHRA-approved. Ozempic (T2DM, 2018), Wegovy (obesity, 2023), Rybelsus (T2DM oral, 2020). Wegovy 7.2 mg approved 12 January 2026.	2026-04-24
US (FDA)	FDA-approved across all three formulations. Wegovy received cardiovascular indication expansion in 2024 (SELECT data).	2026-04-24
EU (EMA)	EMA-approved across all formulations	2026-04-24
NHS England	Wegovy available through Tier 3 weight management services; Ozempic/Rybelsus standard T2DM prescribing	2026-04-24
NHS Scotland, Wales, NI	Similar pattern to England with regional variation	2026-04-24
WADA (sport)	Not on prohibited list	2026-04-24

Narrative. Semaglutide is the most-regulated, longest-approved, widest-available GLP-1 agonist in the UK and globally. The 7.2 mg Wegovy dose (approved UK January 2026) addresses the plateau issue many long-term users have experienced. The cardiovascular indication (2024) is historically significant — first ever for an obesity medication.

For UK readers: semaglutide is legal to obtain and use via prescription from UK-licensed providers. Compounded or grey-market semaglutide is unlicensed medicine under UK law.

What to track in Peptrax

Semaglutide tracking is shaped by the same logic as tirzepatide — structured weight, waist, GI symptoms, appetite — but with a longer titration ladder (0.25 → 0.5 → 1.0 → 1.7 → 2.4 mg for Wegovy) and a more pronounced nausea pattern in the first 4–8 weeks. The most useful daily field early is GI symptoms on a 0–5 scale paired with food notes, because the diet pattern that minimises nausea (smaller portions, lower fat, slower eating) is something users learn empirically over the first month. Logging it as it happens makes that easier than reconstructing it later.

Realistically, the UK access path is private prescription for most users — Numan, Voy, Boots Online, Superdrug Online Doctor, Lloyds, or via a Tier 3 NHS weight-management service for the minority who qualify. Logging the prescriber, monthly cost, and dose schedule is the foundation for the cost-vs-effect conversation. With semaglutide more than tirzepatide, the secondary cost matters too: bloods, follow-up consultations, and dose changes all add up over a 6–12 month course.

The plateau problem is unusually relevant for semaglutide. Many long-term users hit a weight floor at the 2.4 mg dose, and the question of whether to stay, switch to tirzepatide, or step down is genuinely individual. Three to six months of structured weekly weight, waist, and side-effect data is what makes that conversation with a clinician evidence-based rather than impressionistic. Flag any concerning symptoms — persistent severe nausea, signs of pancreatitis or gallbladder issues, vision changes — for prescriber review rather than logging through them.

For personal tracking and informational purposes only — not medical advice.