Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist known under the brand names Ozempic (type 2 diabetes therapy) and Wegovy (weight management). The most common Ozempic side effects documented in the registration clinical trials involve the gastrointestinal (GI) tract: nausea, diarrhoea, vomiting and constipation. Serious adverse events are much rarer, but the manufacturer's approved label contains specific warnings and contraindications. Below is a summary of the documented safety profile of semaglutide from two large randomised studies, STEP 1 (the 2.4 mg dose for weight management) and SUSTAIN-6 (cardiovascular safety in diabetes), along with the key points of the label. This material is a reference and research resource and is not medical advice.

Where the safety data come from

The adverse-event (AE) profile of a GLP-1 agonist is judged not by individual reviews but by controlled studies in which semaglutide is compared with placebo in large samples. Two works became the reference points for understanding what semaglutide side effects look like in numbers:

  • STEP 1 (Wilding et al., NEJM, 2021), 68 weeks, 1961 adults with overweight or obesity without diabetes; semaglutide 2.4 mg subcutaneously once a week compared with placebo alongside lifestyle changes.
  • SUSTAIN-6 (Marso et al., NEJM, 2016), median follow-up 2.1 years, 3297 patients with type 2 diabetes and high cardiovascular risk; doses of 0.5 and 1.0 mg versus placebo.

These two studies complement each other: STEP 1 shows the frequency of typical GI reactions at a high dose, and SUSTAIN-6 the cardiovascular safety and rare signals over long-term use. Most gastrointestinal reactions follow directly from the GLP-1 mechanism of action: the drug slows gastric emptying and suppresses appetite, so nausea and changes in stool are an expected, not an abnormal, effect. A more detailed review of the compound's pharmacology is collected in our semaglutide monograph.

Common side effects: STEP 1 data

The most widespread reactions are gastrointestinal. In STEP 1 (2.4 mg dose) they were mostly mild or moderate in intensity, transient and largely settled over time as adaptation set in, but occurred substantially more often than on placebo.

Adverse event Semaglutide 2.4 mg Placebo
Nausea 44.2% 17.4%
Diarrhoea 31.5% 15.9%
Vomiting 24.8% 6.6%
Constipation 23.4% 9.5%

STEP 1 additionally documented:

  • Discontinuation of therapy due to AEs: 7.0% versus 3.1% on placebo, mostly due to GI reactions specifically (4.5% versus 0.8%).
  • Serious AEs overall: 9.8% versus 6.4%.
  • Acute pancreatitis: 0.2% versus 0%.
  • Gallbladder disorders (including gallstone disease): 2.6% versus 1.2%.

In other words, nausea was the most frequent reaction, experienced by more than four in ten participants on the active drug, versus roughly two in ten on placebo. At the same time, the share who fully stopped taking it because of intolerance stayed relatively small (7%), and most reactions did not progress to serious ones. This is the typical tolerability portrait of a high dose of semaglutide: frequent but mostly manageable GI symptoms at the start and during dose escalation.

Rare but serious risks

Despite a broadly predictable profile, several rare serious events are documented that define the Ozempic risks and make up the warnings section of the label:

  • Pancreatitis. Acute pancreatitis was rare in STEP 1 (0.2%), but it remains a separate warning for the whole class of GLP-1 agonists and needs attention with the relevant symptoms.
  • Gallbladder disease. The frequency of gallbladder disorders was higher on semaglutide (2.6% versus 1.2%). An additional factor is the rapid weight loss itself, which is an independent risk factor for gallstone formation.
  • GI motility signals (gastroparesis, ileus). In September 2023 the FDA (US Food and Drug Administration) added a mention of intestinal obstruction (ileus) to the Ozempic label based on post-marketing reports. Delayed gastric emptying (gastroparesis-like manifestations) also features in pharmacovigilance.
  • Diabetic retinopathy complications. In SUSTAIN-6 they occurred more often on semaglutide: 3.0% versus 1.8% (hazard ratio 1.76; 95% CI 1.11–2.78; p=0.02). The likely mechanism is a rapid drop in glycaemia in patients with diabetes.
  • C-cell thyroid tumours. The label carries a boxed warning: in rodents semaglutide caused C-cell tumours, including medullary thyroid cancer; whether these data are relevant to humans has not yet been established.
  • "Ozempic face." This is not a direct pharmacological action of the drug but a consequence of rapid weight loss: along with fat tissue, the volume of subcutaneous facial fat decreases, which makes the face look more gaunt.

Cardiovascular safety: SUSTAIN-6

The question of Ozempic safety for the heart and vessels was key for registration, since the drug is taken long-term by patients at high cardiovascular risk. In SUSTAIN-6 the primary composite endpoint (cardiovascular death, non-fatal myocardial infarction or non-fatal stroke) occurred in 6.6% of patients on semaglutide versus 8.9% on placebo, hazard ratio 0.74 (95% CI 0.58–0.95; p<0.001 for non-inferiority). Non-fatal stroke was rarer (1.6% versus 2.7%), and new or progressive nephropathy also rarer (3.8% versus 6.1%). So in this high-risk population cardiovascular events did not rise. The main warning that emerged specifically from this study was the ophthalmological signal (retinopathy) described above.

Contraindications and warnings (per the label)

The approved label for the authorised brands sets out these key restrictions:

  • Contraindications: personal or family history of medullary thyroid cancer (MTC); multiple endocrine neoplasia type 2 (MEN 2); a history of serious hypersensitivity to semaglutide.
  • Warnings and caution: a history of pancreatitis; gallbladder disease; diabetic retinopathy; risk of acute kidney injury against a background of dehydration (from vomiting or diarrhoea); hypoglycaemia when combined with insulin or sulfonylureas; intestinal obstruction (ileus).

What this means for a safety assessment

The documented profile of semaglutide is fairly predictable: dose-dependent GI reactions dominate, and serious events are rare. However, any correct interpretation of tolerability data is possible only when the identity and purity of the studied substance are reliably known. For laboratory use this is critical: impurities, degradants or an inexact active-substance content distort any conclusions about safety and reproducibility. That is why for research purposes we provide research-grade semaglutide with an accompanying certificate of analysis (COA) that records the purity and identity of the batch.

It is important to separate the contexts. Ozempic and Wegovy are approved medicines; their prescription, dose selection and safety monitoring are solely within a doctor's competence, and this article is not an instruction for self-administration. The research compound is supplied for laboratory research and is not a means for treating or preventing disease in humans.

In brief

  • The most common semaglutide side effects are gastrointestinal: nausea (44.2%), diarrhoea (31.5%), vomiting (24.8%), constipation (23.4%) at the 2.4 mg dose in STEP 1.
  • Discontinuation due to AEs, 7.0%; serious AEs, 9.8%; reactions mostly mild/moderate and transient.
  • Rare serious risks: pancreatitis, gallbladder disease, GI motility signals (gastroparesis/ileus), retinopathy complications (SUSTAIN-6: 3.0% versus 1.8%).
  • A boxed warning about C-cell thyroid tumours; contraindications with MTC and MEN 2.
  • "Ozempic face," a consequence of rapid weight loss, not a direct action of the drug.
  • The cardiovascular profile in SUSTAIN-6 is favourable (6.6% versus 8.9%).