Semaglutide for weight loss
The best-evidenced compound on this site — and the only one shown to prevent heart attacks, not just move a scale.
What is being claimed
That once-weekly semaglutide at 2.4 mg produces large, sustained weight loss in adults with obesity, and that the loss is clinically meaningful rather than cosmetic.
This is one of the few claims on this site that reaches four dots. The trials are large, long, placebo-controlled, published in the highest-tier journals, and the effect has been confirmed against a hard clinical endpoint rather than a scale reading alone.
The trial record
Principal randomised, placebo-controlled trials
STEP 1 is the anchor. Adults with obesity and no diabetes lost a mean of 14.9% of body weight over 68 weeks against 2.4% on placebo — a placebo-adjusted difference of 12.4 percentage points, with 86% of participants losing at least 5%.1
SELECT is what separates semaglutide from almost everything else this publication grades. It enrolled 17,604 adults with established cardiovascular disease and overweight or obesity but without diabetes, and asked whether the drug prevents heart attacks and strokes. It did: 6.5% of the semaglutide group had a major adverse cardiovascular event against 8.0% on placebo — a hazard ratio of 0.80 (95% CI 0.72–0.90, P<0.001).2
That result matters beyond cardiology. Most weight-loss evidence measures weight, and weight is a surrogate — a stand-in for the outcomes people actually care about. SELECT tested one of those outcomes directly, in a population large enough to detect it. Very few compounds on this site have ever been examined that way, and none of the supplements have.
Who ran the trials
We grade a claim partly on how many independent groups have examined it, so this needs saying plainly: the overwhelming majority of semaglutide's trial evidence was generated by Novo Nordisk, which makes and sells the drug. STEP, SUSTAIN, SELECT and OASIS are all company programmes.
Independent replication does exist, and it is what lifts this claim past Moderate. The HISTORI trial was investigator-initiated by Danish clinicians in community mental-health services, testing semaglutide in antipsychotic-treated patients with schizophrenia, prediabetes and obesity.3 A separate academic trial at a US medical centre randomised adults with alcohol use disorder to low-dose semaglutide or placebo.4 Neither was a Novo Nordisk study.
One caveat a careful reader deserves. HISTORI's funders included the Novo Nordisk Foundation — a body legally distinct from Novo Nordisk A/S, but one whose holdings control the company that sells semaglutide. Investigator-initiated is not the same as independently funded, and in Danish diabetes research the distinction is frequently blurred by exactly this route.
We still count it. The investigators designed, ran and reported the trial without the manufacturer, and the finding is consistent with the academic alcohol-use-disorder trial, which has no such tie. But readers weighing "has anyone outside the company checked this?" should know how the money reached the room.
Safety and unknowns
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Boxed warning — thyroid C-cell tumours
Semaglutide causes dose-dependent thyroid C-cell tumours in rodents. Whether this translates to humans is unknown. The drug is contraindicated in anyone with a personal or family history of medullary thyroid carcinoma or MEN 2.6 Best available human evidence has not shown an increase in common thyroid cancers.
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NAION — sudden, irreversible vision loss
In 2025 the EMA's pharmacovigilance committee concluded that non-arteritic anterior ischaemic optic neuropathy is a very rare side effect of semaglutide — up to 1 in 10,000 — and required label updates across Ozempic, Rybelsus and Wegovy.7 The WHO issued a global alert the following month.8 It is rare, it is serious, and it does not reverse. Sudden vision change is a stop-and-call-a-doctor event.
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Gastrointestinal effects
Nausea, vomiting, diarrhoea and constipation, concentrated during dose escalation and the most common reason for discontinuation.1
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Lean mass
A meaningful fraction of the weight lost is lean tissue rather than fat. This is a known feature of rapid weight loss generally, not unique to this drug, and it is the strongest argument for resistance training and adequate protein alongside treatment.
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Regain on discontinuation
The STEP 1 extension found participants regained roughly two-thirds of the lost weight within a year of stopping. The effect is a treatment, not a cure.
What is actually in the vial
Semaglutide is approved and prescribable, which makes it unusual among the compounds we cover. But a large parallel market sells "semaglutide" outside that channel — compounded, research-labelled, or imported. The trials above describe a licensed product of known identity and concentration, administered at a titrated dose. They say nothing about material from an unregulated vendor, and dosing errors with non-standard concentrations are a documented source of harm.
How the grade was reached
Evidence strength — Strong, criteria checked
The criterion doing the most work here is two independent groups plus a published systematic review. Semaglutide clears it, but not by a wide margin — the independent trials are far smaller than the company's, and one of them carries the funding tie described above. Four dots means the question has been examined thoroughly; it does not mean the examination was evenly distributed.
Bottom line
Semaglutide works, and the evidence that it works is better than for anything else on this site. Nearly fifteen per cent mean weight loss over 68 weeks against placebo, replicated, pooled, and — uniquely among the compounds we grade — shown to reduce heart attacks and strokes in a trial of more than seventeen thousand people.
The honest qualifications are that the evidence base is dominated by its manufacturer, that a rare and irreversible eye complication was added to its label in 2025, that it carries a rodent-derived boxed warning nobody has resolved in humans, and that the weight returns when you stop.
None of that changes the grade. All of it belongs next to the grade.
Sources
- Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine. 2021;384(11):989–1002. PMID 33567185. pubmed.ncbi.nlm.nih.gov/33567185
- Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT). New England Journal of Medicine. 2023;389(24):2221–2232. PMID 37952131. pubmed.ncbi.nlm.nih.gov/37952131
- Semaglutide treatment of antipsychotic-treated patients with schizophrenia, prediabetes, and obesity: the HISTORI randomized clinical trial. 2025. Investigator-initiated; funders included the Novo Nordisk Foundation. PMID 40900607. pubmed.ncbi.nlm.nih.gov/40900607
- Hendershot CS, Bremmer MP, Paladino MB, et al. Once-weekly semaglutide in adults with alcohol use disorder: a randomized clinical trial. JAMA Psychiatry. 2025. PMID 39937469. pubmed.ncbi.nlm.nih.gov/39937469
- Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). New England Journal of Medicine. 2025. Open-label; funded by Eli Lilly. PMID 40353578. pubmed.ncbi.nlm.nih.gov/40353578
- US Food and Drug Administration. Semaglutide prescribing information — boxed warning, risk of thyroid C-cell tumours. accessdata.fda.gov
- European Medicines Agency. PRAC concludes eye condition NAION is a very rare side effect of semaglutide medicines Ozempic, Rybelsus and Wegovy. 2025. ema.europa.eu
- World Health Organization. The use of semaglutide medicines and risk of non-arteritic anterior ischemic optic neuropathy (NAION). 27 June 2025. who.int
