Pooled across the STEP 1, 2 and 3 trials, 2117 adults taking semaglutide 2.4 mg for 68 weeks reported nausea (43.9% against 16.1% on placebo), diarrhoea (29.7% against 15.9%), vomiting (24.5% against 6.3%) and constipation (24.2% against 11.1%) (PMID 34514682). Of all gastrointestinal events recorded on semaglutide, 98.1% were mild or moderate and 99.5% were non-serious.
The rates get quoted everywhere. The timing almost never does, and it is the more useful half. In the same pooled analysis the median nausea episode lasted 8 days, vomiting 2 days and diarrhoea 3 days, and the cumulative incidence of a first gastrointestinal event flattened out after week 20. This article follows that clock.
- Nausea was the most common event at 43.9% against 16.1% on placebo, followed by diarrhoea, vomiting and constipation.
- Median durations were 8 days for nausea, 3 for diarrhoea and 2 for vomiting. Constipation was the outlier at 47 days.
- New events clustered in the 16-week escalation period and the cumulative incidence plateaued after week 20.
- 4.3% of semaglutide participants stopped permanently because of a gastrointestinal event, and 98.1% of those events were mild or moderate.
The four symptoms, and how long each lasted
These figures come from one pooled safety analysis of STEP 1 to 3, covering 2117 participants randomised to semaglutide 2.4 mg and 1262 to placebo for 68 weeks. Durations are medians, so half of episodes were shorter.
| Symptom | Semaglutide 2.4 mg | Placebo | Median duration on semaglutide |
|---|---|---|---|
| Nausea | 43.9% | 16.1% | 8 days |
| Diarrhoea | 29.7% | 15.9% | 3 days |
| Vomiting | 24.5% | 6.3% | 2 days |
| Constipation | 24.2% | 11.1% | 47 days |
| Any gastrointestinal event | 72.9% | 47.1% | Not applicable |
Two things stand out. The placebo column is not empty, which is why a symptom list without a comparator tells you very little. And constipation behaves differently from the rest: at a median of 47 days on semaglutide against 35 on placebo, it is the one event in the table that is slow rather than sharp.
When the events happen
Semaglutide is escalated from 0.25 mg to the 2.4 mg maintenance dose over 16 weeks, in four-week steps. Almost all of the new gastrointestinal events arrive inside that window.
| Phase | Weeks | What the trials recorded |
|---|---|---|
| Escalation | 1 to 16 | Events reported most frequently here. STEP 1 described nausea and diarrhoea as typically transient and mild to moderate, subsiding with time. |
| Reaching maintenance | 17 to 20 | Cumulative incidence of a first gastrointestinal event plateaued after week 20 in the pooled STEP 1 to 3 analysis. |
| Maintenance | After week 20 | In STEP 4, semaglutide 2.4 mg maintenance was well tolerated, and almost all permanent discontinuations for gastrointestinal events had already happened during the run-in. |
STEP 4 is the informative one here, because it ran a 20-week open escalation before randomising anyone. Of those enrolled in that run-in, 11.0% never reached the 2.4 mg target dose, and adverse events were the most common reason, accounting for 48 of the 99. Permanent discontinuation for gastrointestinal events affected 4.9% of enrolled participants and occurred almost entirely in that run-in period.
So the attrition is front-loaded. The people still on 2.4 mg at week 30 are, by definition, the ones who tolerated getting there. That is worth remembering when you read a maintenance-phase tolerability figure as though it applied to someone about to start. The schedule itself is laid out in our semaglutide dosage chart.
The same drug in different populations
Rates shift with who was enrolled and at what dose, which is the second reason a single percentage travels badly.
| Trial | Dose and population | Any adverse event | Gastrointestinal events |
|---|---|---|---|
| STEP 2 (PMID 33667417) | 2.4 mg, type 2 diabetes, 68 weeks | 87.6% vs 76.9% placebo | 63.5% vs 34.3% placebo |
| STEP 2 (PMID 33667417) | 1.0 mg, type 2 diabetes, 68 weeks | 81.8% | 57.5% |
| STEP 6 (PMID 35131037) | 2.4 mg, east Asian adults, 68 weeks | 86% vs 79% placebo | 59% vs 30% placebo |
| SUSTAIN FORTE (PMID 34293304) | 2.0 mg, type 2 diabetes, 40 weeks | Not reported this way | 34% vs 31% on 1.0 mg |
The STEP 2 rows are the cleanest dose comparison available inside a single trial: 2.4 mg against the 1.0 mg diabetes dose in the same population gave 63.5% against 57.5% for gastrointestinal events, a gap of six percentage points for a dose more than twice the size. SUSTAIN FORTE found much the same when it compared 2.0 mg with 1.0 mg over 40 weeks, at 34% against 31%.
Discontinuation stayed low across all of them. STEP 1 reported 4.5% of semaglutide participants stopping because of gastrointestinal events against 0.8% on placebo (PMID 33567185). STEP 6 reported adverse events leading to discontinuation in 3% of its 2.4 mg group.
See the upside these rates are weighed against
The same trials recorded the weight change. Put your own starting weight against those percentages.
Feeling sick is not the mechanism
The pooled analysis also tested whether the weight loss runs through the nausea. It does not. Mean weight loss on semaglutide 2.4 mg was similar in participants without gastrointestinal events (9.6% to 17.1% across the three trials) and with them (11.4% to 17.7%). A mediation analysis found that of the additional 7.6 to 14.4 percentage points of weight loss over placebo, under one percentage point was mediated by gastrointestinal events.
The equivalent analysis of the tirzepatide trials reached the same answer, and we go through it in tirzepatide side effects. Neither drug is working by making people too queasy to eat.
The risks that frequency tables miss
Common and serious are different questions. The semaglutide label carries:
- A boxed warning for thyroid C-cell tumours, based on rodent studies, with the human relevance not established. It is contraindicated in anyone with a personal or family history of medullary thyroid carcinoma or with multiple endocrine neoplasia syndrome type 2.
- Acute pancreatitis, including fatal and non-fatal haemorrhagic or necrotising cases reported with GLP-1 receptor agonists.
- Acute gallbladder disease.
- Acute kidney injury, which can follow dehydration from vomiting or diarrhoea.
- Hypoglycaemia when used alongside insulin or an insulin secretagogue.
- Diabetic retinopathy complications in people with type 2 diabetes.
- Hypersensitivity reactions, including anaphylaxis and angioedema.
- Monitoring for depression, suicidal thoughts and behaviour.
Severe or persistent abdominal pain, persistent vomiting, or signs of an allergic reaction are reasons to seek medical attention rather than to wait out a median duration.
What this evidence does not cover
- Weeks beyond 68 or 104. The pooled analysis stops where the trials stop.
- Compounded semaglutide. None of these trials used a compounded product, so neither the dose delivered nor the event rate is described by them.
- The 7.2 mg dose. STEP UP tested it separately, and its tolerability profile is not the one pooled above. We cover that trial in how much weight you can lose on Wegovy.
FAQ
How long do semaglutide side effects last?
In the pooled STEP 1 to 3 analysis the median episode lasted 8 days for nausea, 3 days for diarrhoea and 2 days for vomiting. Constipation was the exception at a median of 47 days. New events also became much less likely after week 20, once the dose stopped rising.
What are the most common side effects of semaglutide?
Nausea, reported by 43.9% of participants on semaglutide 2.4 mg against 16.1% on placebo, then diarrhoea at 29.7%, vomiting at 24.5% and constipation at 24.2%. In total, 72.9% of semaglutide participants reported at least one gastrointestinal event, against 47.1% on placebo.
Do the side effects get better over time?
That is what the trials recorded. STEP 1 described nausea and diarrhoea as typically transient and subsiding with time, the pooled analysis found the cumulative incidence of a first event plateauing after week 20, and STEP 4 found maintenance dosing at 2.4 mg well tolerated once participants had completed escalation.
How many people stop semaglutide because of side effects?
STEP 1 reported 4.5% stopping for gastrointestinal events against 0.8% on placebo. The pooled STEP 1 to 3 figure was 4.3%. In the STEP 4 run-in, 11.0% never reached the 2.4 mg target dose, with adverse events the most common reason.
Are side effects worse at a higher dose?
Less than the dose difference suggests. In STEP 2, gastrointestinal events were reported by 63.5% on 2.4 mg and 57.5% on 1.0 mg in the same population. SUSTAIN FORTE recorded 34% on 2.0 mg against 31% on 1.0 mg over 40 weeks.
Does having side effects mean the drug is working?
No. Weight loss was similar with and without gastrointestinal events, and a mediation analysis attributed under one percentage point of the additional weight loss over placebo to those events.
This article reports what the published semaglutide trials and the product label record. It is not medical advice, and nothing here is a reason to start, change or stop a dose without your prescriber.
Sources
- Wharton S, Calanna S, Davies M, et al. Gastrointestinal tolerability of once-weekly semaglutide 2.4 mg in adults with overweight or obesity, and the relationship between gastrointestinal adverse events and weight loss. <em>Diabetes Obes Metab</em>. 2022;24(1):94-105. PMID: 34514682
- Wilding JPH, Batterham RL, Calanna S, et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1). <em>N Engl J Med</em>. 2021;384(11):989-1002. PMID: 33567185
- Davies M, Færch L, Jeppesen OK, et al. Semaglutide 2·4 mg once a week in adults with overweight or obesity, and type 2 diabetes (STEP 2). <em>Lancet</em>. 2021;397(10278):971-984. PMID: 33667417
- Kadowaki T, Isendahl J, Khalid U, et al. Semaglutide once a week in adults with overweight or obesity, with or without type 2 diabetes in an east Asian population (STEP 6). <em>Lancet Diabetes Endocrinol</em>. 2022;10(3):193-206. PMID: 35131037
- Frías JP, Auerbach P, Bajaj HS, et al. Efficacy and safety of once-weekly semaglutide 2·0 mg versus 1·0 mg in patients with type 2 diabetes (SUSTAIN FORTE). <em>Lancet Diabetes Endocrinol</em>. 2021;9(9):563-574. PMID: 34293304
- Wegovy (semaglutide) and Ozempic (semaglutide) United States prescribing information, Novo Nordisk, for the boxed warning, contraindications and warnings.