Semaglutide

Semaglutide is a synthetic analogue of glucagon-like peptide-1 (GLP-1), a gut hormone released after eating. Natural GLP-1 is broken down within minutes. Novo Nordisk’s chemists extended its life by swapping two amino acids (Aib at position 8 and arginine at position 34) and attaching a fatty-acid side chain at lysine 26, which binds the peptide to albumin in the blood; the 2015 discovery paper describes how this was designed to allow once-weekly dosing ^[1].

It is sold as three branded medicines: Ozempic (weekly injection for type 2 diabetes), Rybelsus (daily tablet for type 2 diabetes) and Wegovy (weekly injection, and since December 2025 in the US a daily tablet, for weight management) ^[11]. These are made under pharmaceutical manufacturing standards, supplied in fixed-dose pens or tablets, and prescribed by a clinician.

Semaglutide is also widely offered by online vendors as a freeze-dried powder labelled “for research use only”. That product is not Ozempic or Wegovy. It is made by unknown manufacturers, is not batch-released by a regulator, and its identity and dose can only be checked by independent testing. Our guide to what research peptides are explains why the label matters. Semaglutide is often compared with the dual agonist tirzepatide and the investigational triple agonist retatrutide.

Semaglutide activates the GLP-1 receptor, which is found in the pancreas, the gut and the brain. In the pancreas this increases insulin release and reduces glucagon release when blood glucose is raised, which is why it lowers blood sugar in type 2 diabetes. It also slows stomach emptying and acts on appetite centres in the brain, reducing hunger and food intake; this appetite effect is the main driver of the weight loss seen in trials.

The albumin binding described in the discovery paper is what gives semaglutide its long half-life ^[1]. In people, the half-life of semaglutide 2.4 mg was measured at about 145 to 165 hours, roughly a week, which allows a single weekly injection ^[12]. The weight-management dose (2.4 mg weekly) is higher than the doses used in the diabetes outcome trials described below (0.5 mg or 1.0 mg weekly).

The pivotal obesity trial, STEP 1, enrolled 1,961 adults with a BMI of 30 or more (or 27 or more with a weight-related condition) who did not have diabetes. Over 68 weeks, participants on semaglutide 2.4 mg once weekly plus lifestyle advice lost an average of 14.9% of body weight, against 2.4% on placebo; about half of the semaglutide group lost 15% or more ^[2].

STEP 4 asked what happens when treatment stops. After everyone took semaglutide for a 20-week run-in (losing an average of 10.6%), 803 participants were randomised to continue or switch to placebo. Over the next 48 weeks, those who continued lost a further 7.9%, while those switched to placebo regained 6.9% ^[3]. This is the main trial evidence that weight tends to return after stopping.

In a head-to-head trial in adults with obesity but without diabetes (SURMOUNT-5), tirzepatide at its maximum tolerated dose produced greater weight loss at 72 weeks than semaglutide at its maximum tolerated dose: 20.2% against 13.7% ^[8].

SUSTAIN-6 tested semaglutide 0.5 mg or 1.0 mg weekly in 3,297 people with type 2 diabetes at high cardiovascular risk. Over 104 weeks, the combined rate of cardiovascular death, non-fatal heart attack or non-fatal stroke was 6.6% with semaglutide and 8.9% with placebo. The same trial found more diabetic eye (retinopathy) complications in the semaglutide group ^[4].

SELECT extended this to people without diabetes. It enrolled 17,604 adults aged 45 or over with existing cardiovascular disease and a BMI of 27 or more. Over a mean follow-up of about 40 months, major cardiovascular events occurred in 6.5% on semaglutide 2.4 mg against 8.0% on placebo, a 20% relative reduction ^[5].

FLOW studied semaglutide 1.0 mg weekly in 3,533 people with type 2 diabetes and chronic kidney disease. The trial was stopped early for efficacy: the risk of major kidney disease events was 24% lower with semaglutide, and death from any cause was 20% lower ^[6].

In type 2 diabetes, the SURPASS-2 trial compared semaglutide 1 mg with tirzepatide. Semaglutide lowered HbA1c by 1.86 percentage points over 40 weeks; all three tirzepatide doses lowered it further ^[7].

Unlike most peptides covered on this site, semaglutide has been studied in tens of thousands of people in randomised, placebo-controlled trials published in journals such as the New England Journal of Medicine and JAMA. The trials above are a small selection. The evidence applies to the licensed products used at the licensed doses under medical supervision; it does not tell you anything about the content or purity of an unlicensed vial bought online.

The STEP obesity trials used 2.4 mg by subcutaneous injection once weekly, reached by gradual dose escalation (in STEP 4 the escalation took 16 weeks) ^{[2] [3]}. SELECT also used 2.4 mg weekly ^[5]. The diabetes outcome trials used lower weekly doses: 0.5 mg or 1.0 mg in SUSTAIN-6 and 1.0 mg in FLOW ^{[4] [6]}.

These are the doses studied in trials of the licensed medicine, reported here for information only. They are not a recommendation, and they cannot be transferred to research powders whose actual peptide content is unknown. Our peptide calculator explains reconstitution arithmetic but cannot correct for a mislabelled vial.

The most common side effects in trials were gastrointestinal: nausea, diarrhoea, vomiting and constipation. In STEP 1 these were usually mild to moderate and eased over time, but 4.5% of semaglutide users stopped treatment because of gastrointestinal events, against 0.8% on placebo ^[2]. In SELECT, 16.6% on semaglutide and 8.2% on placebo permanently stopped the study drug because of adverse events ^[5].

SUSTAIN-6 found a higher rate of diabetic retinopathy complications with semaglutide (hazard ratio 1.76) in people with type 2 diabetes ^[4]. Prescribing information for GLP-1 medicines also warns about pancreatitis, gallbladder problems and, in the US, carries a boxed warning about thyroid C-cell tumours seen in rodents. Anyone taking the licensed medicine should follow their prescriber’s advice.

Unlicensed products add further risks. A 2024 study that bought semaglutide from illegal online pharmacies found that all three vials delivered were probably substandard or falsified: measured purity was only 7.7% to 14.4% against a claimed 99%, and bacterial endotoxin was detected in every sample. None of the three Ozempic pens ordered arrived ^[9].

Semaglutide is an approved prescription-only medicine. In the US it is licensed as Ozempic and Rybelsus for type 2 diabetes and Wegovy for chronic weight management; the FDA approved the oral Wegovy tablet in December 2025 ^[11]. It is also licensed in the UK, the EU and many other countries. Licensed use requires a prescription; buying unlicensed “research” semaglutide for personal injection is a different matter, and the rules on sale and possession vary by country, as set out on our legal status pages.

Semaglutide is not on the WADA Prohibited List. WADA added it to its Monitoring Program in 2024, which tracks use in sport without banning it ^[10].

Licensed semaglutide pens are kept in a refrigerator before first use and must not be frozen; the patient leaflet for each product sets out how long it can then be kept at room temperature. Research-grade semaglutide is usually a freeze-dried powder, which is generally kept cold, dry and away from light, and refrigerated once reconstituted. See our guides to storing peptides and bacteriostatic water.

PepFinder does not sell semaglutide. If you are comparing research suppliers, you can compare prices, learn how to read a COA, and see which vendors publish third-party tested batches. Given the purity results above, an independent certificate that confirms identity and content is the minimum to look for.