Safety reference
Semaglutide Weight Loss Side Effects: What the Labels Actually List
Every figure on this page comes from the FDA-approved prescribing information, with the placebo rate beside it — because a side effect is only meaningful compared to what happens without the drug.
- 73%Of people on Wegovy reported a gastrointestinal effect of some kind
- 44% / 16%Nausea on the drug, against nausea on placebo
- RodentsThe species in which the boxed-warning tumours were actually seen
- EscalationWhen most effects appear, and when most of them settle
The short version
Most GLP-1 side effects are gastrointestinal, most are mild to moderate, and most arrive while the dose is being increased rather than later. That is the accurate summary, and it is also the one most likely to be used to wave the question away. The numbers underneath it are worth seeing.
Nearly three quarters of people taking Wegovy in its trials reported some gastrointestinal effect. That is a large number and it is not hidden — it is in the approved labeling. What the labeling also shows is that a substantial minority of people on placebo reported the same things, which is the comparison most coverage leaves out.
Semaglutide weight loss side effects, with the placebo rate
| Effect | Wegovy | Placebo |
|---|---|---|
| Any gastrointestinal effect | 73% | — |
| Nausea | 44% | 16% |
| Diarrhoea | 30% | — |
| Vomiting | 25% | 6% |
Read the placebo column before drawing conclusions. Sixteen percent of people who took an inactive injection reported nausea. That does not mean the drug’s nausea is imaginary — the difference is large and real — but it does mean that roughly a third of the nausea reported on semaglutide would have been reported anyway.
Beyond those four, the labeling lists everything reported by at least 5% of people: constipation, abdominal pain, headache, fatigue, indigestion, dizziness, abdominal bloating, belching, flatulence, gastroenteritis, reflux, and low blood sugar in people who also have type 2 diabetes.
How long do semaglutide side effects last?
The consistent finding across trials of this drug class is that gastrointestinal effects cluster during dose escalation — the weeks when the amount is being stepped up — and diminish rather than persist. In the head-to-head trial of tirzepatide against semaglutide, adverse events in both groups were described as mostly mild to moderate and occurring primarily during escalation. The higher-dose semaglutide trial reported the same pattern: effects diminished over time.
That is a pattern, not a promise. Trials report averages across large groups, and a group in which most people settle within weeks can still contain people who do not. What the evidence supports is that the early weeks are the hardest and that this usually improves — not that it improves for everyone, and not on a timetable anyone can quote you.
Our scope
We do not publish titration schedules or advice on managing dose. That is between you and the prescriber who wrote your prescription, and it is the single most common thing this class of drug gets wrong when handled outside a clinical relationship.
The boxed warning, read carefully
Both semaglutide and tirzepatide carry a boxed warning — the FDA’s most serious label warning — about thyroid C-cell tumours. It is the thing people find when they search, and it is routinely reported without the detail that determines what it means.
What the label actually says
The tumours were seen in rats. Tirzepatide’s labeling states that whether it causes thyroid C-cell tumours in humans, including medullary thyroid carcinoma, is unknown — the human relevance of the rodent finding has not been determined.
That is neither reassurance nor alarm. It means a real signal was found in a rodent study at clinically relevant exposures, and that nobody has established whether it transfers to people. Both drugs are contraindicated in anyone with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 — a specific exclusion, not a general caution, and one worth raising before a first prescription rather than after.
The other item worth knowing is quieter and more common. The labeling notes that most reported cases of acute kidney injury occurred in people who had gastrointestinal effects severe enough to cause dehydration. The mechanism is ordinary: sustained vomiting or diarrhoea, reduced fluid intake, and kidneys under strain. It is the most plausible route by which a common side effect becomes a serious one.
Tirzepatide vs semaglutide side effects
The two drugs’ labeled effects are close to identical in kind. Zepbound’s most common reactions are nausea, diarrhoea, vomiting, constipation, abdominal pain, indigestion, injection site reactions, fatigue, hypersensitivity reactions, belching, hair loss and reflux. Set against Wegovy’s list, the overlap is almost complete — injection site reactions and hair loss are the notable additions.
The only trial that compared them directly found gastrointestinal effects most common in both arms, mostly mild to moderate, and concentrated during escalation. It did not report one drug as meaningfully better tolerated than the other. Anyone telling you that one is definitively easier on the stomach is going beyond what that trial showed. What it did establish is a difference in weight loss and in price, which is a different question.
Frequently asked questions
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When do semaglutide side effects start?
Trials consistently report them clustering during dose escalation rather than at a single point. That means the weeks after starting, and the weeks after each increase, are when most people notice them.
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What are the long-term side effects of semaglutide?
Not fully known, and anyone who tells you otherwise is overreaching. The pivotal obesity trials ran 68 to 72 weeks. That is long enough to characterise common effects and far too short to describe what a decade of continuous use does. The labeling reflects what has been observed, not everything that could be.
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Are the side effects worse on compounded semaglutide?
There is no equivalent labeling to quote, because compounded products have not been through approval. The FDA has issued a specific alert about dosing errors with compounded injectable semaglutide, some serious enough to require hospitalisation, arising because concentrations vary between pharmacies. See our page on compounded semaglutide.
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Should the boxed warning stop me taking it?
That is a question for a prescriber, and it turns on your own history. What we can say is what the labeling says: the tumours were found in rats, human relevance is undetermined, and the drugs are contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2.
Sources and method
- Semaglutide frequencies and the ≥5% list. Wegovy (semaglutide) injection, FDA-approved prescribing information.
- Tirzepatide effects, boxed warning and contraindications. Zepbound (tirzepatide) injection, FDA-approved prescribing information.
- Timing and the head-to-head comparison. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity, New England Journal of Medicine, 2025 (SURMOUNT-5).
- What we have not done. We have not reproduced the full adverse reactions table from either label — the figures above are the headline frequencies, and the complete tables are in the linked documents. The diarrhoea placebo rate was not legible in the source we consulted and is marked as unavailable rather than estimated. We publish no dosing or titration guidance of any kind.
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Verified against the approved labeling 30 August 2026. We recheck this page whenever either label is revised.