GLP-1 side effects: what’s common and what’s serious
The short answer
The great majority of GLP-1 side effects are digestive — nausea, constipation, diarrhoea, reflux, burping — and they are usually worst in the days after starting or increasing a dose, then ease as your body adjusts.
A small number of reactions are serious and need prompt medical attention. The most important to recognise are severe, persistent abdominal pain, repeated vomiting with signs of dehydration, and symptoms of a gallbladder problem.
How common are they, exactly?
Most articles say these effects are “common” without saying how common. The figures below come from the FDA-approved prescribing information for each product — the same trials the approvals were based on. The placebo column matters: some of what people attribute to the drug happens anyway.
| Effect | Wegovy 2.4 mgsemaglutide | Placebo(Wegovy trials) | Zepbound 15 mgtirzepatide | Placebo(Zepbound trials) |
|---|---|---|---|---|
| Nausea | 44% | 16% | 28% | 8% |
| Diarrhoea | 30% | 16% | 23% | 8% |
| Vomiting | 24% | 6% | 13% | 2% |
| Constipation | 24% | 11% | 11% | 5% |
| Abdominal pain | 20% | 10% | 10% | 5% |
| Headache | 14% | 10% | — | — |
| Fatigue | 11% | 5% | — | — |
| Injection-site reaction | — | — | 8% | 2% |
Two things are worth drawing out. First, nausea is by far the most common, affecting roughly two in five people on semaglutide at the weight-management dose. Second, the drugs are not identical: in their respective trials, semaglutide showed higher rates of nausea, vomiting and constipation than tirzepatide, though the trials were separate and not designed to be compared head-to-head, so treat that as a rough signal rather than a verdict.
These effects are dose-related: they typically flare after a dose increase and settle within days to weeks. Because they come from the medication slowing your stomach, the practical levers are the same for all of them — smaller meals, less fat and fried food, eating slowly, stopping at the first sign of fullness, and not rushing dose increases.
Discontinuation because of side effects is much less common than the effects themselves. In the Wegovy trials, the reactions that most often led people to stop were nausea (1.8% versus 0.2% on placebo), vomiting (1.2% versus 0%) and diarrhoea (0.7% versus 0.1%) — so the great majority of people who experience these effects continue treatment.
Because these effects come from the medication slowing your stomach, the practical levers are the same across all of them: smaller meals, less fat and fried food, eating slowly, stopping at the first sign of fullness, and not rushing dose increases.
Get medical help urgently if you have
- Severe, persistent abdominal pain, especially pain radiating to your back, with or without vomiting — this can signal pancreatitis. Stop the medication and seek care.
- Pain in the upper-right abdomen, fever, yellowing of the eyes or skin, or pale stools — possible gallbladder or bile-duct problems, which occur more often with rapid weight loss.
- Vomiting you cannot keep ahead of, or signs of dehydration such as dizziness, very dark urine, or passing little urine — dehydration can injure the kidneys.
- A lump or swelling in your neck, hoarseness, trouble swallowing, or shortness of breath — report these promptly.
- Signs of a serious allergic reaction — swelling of the face, lips, tongue or throat, or difficulty breathing. Call emergency services.
- Sudden changes in vision, particularly if you have diabetic retinopathy.
Boxed warning and who should not take these
Semaglutide and tirzepatide products carry a boxed warning — the FDA’s most serious labeling warning — regarding thyroid C-cell tumours observed in rodent studies. Whether this applies to humans has not been established, but as a result these medications are contraindicated if you or a family member has a history of medullary thyroid carcinoma, or if you have Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).
Tell your prescriber before starting if you have any history of:
- pancreatitis;
- gallbladder disease;
- severe gastrointestinal disease, including gastroparesis;
- diabetic retinopathy;
- kidney disease;
- type 1 diabetes — these medications are not a substitute for insulin.
These medications are not recommended during pregnancy, and planning ahead matters more than people expect. Because semaglutide has a long half-life, the Wegovy prescribing information advises discontinuing it at least two months before a planned pregnancy. If you are pregnant, trying to conceive, or breastfeeding, raise this with your prescriber before starting.
Surgery, procedures and anaesthesia
Because GLP-1 medications slow stomach emptying, food may remain in the stomach longer than expected before a procedure requiring sedation or general anaesthesia. Guidance in this area has been actively evolving. Always tell your surgeon and anaesthetist that you take a GLP-1 medication, well before the procedure, and follow their instructions on whether to pause it.
Individual side-effect guides
- ConstipationWhy it happens and what actually helps.
- Hair lossUsually the weight loss, not the drug — and usually temporary.
More guides in this section are being written and will be added as they are completed.
Sources
Each source below supports specific statements on this page. We cite the strongest available authority and verify against the current version before publishing.
- FDA-approved prescribing information (drug labels)DailyMed, U.S. National Library of MedicineSupports: Labeled adverse reactions, warnings, and contraindications for GLP-1 products.
- Drug approvals, labeling and safety communicationsU.S. Food & Drug AdministrationSupports: Boxed warnings and post-marketing safety communications.
- Diabetes, obesity and digestive-disease informationNational Institute of Diabetes and Digestive and Kidney Diseases (NIH)Supports: Background on gastrointestinal effects and metabolic treatment.
- Clinical guidance on gastrointestinal effectsAmerican Gastroenterological AssociationSupports: Clinical guidance on gastrointestinal effects of GLP-1 receptor agonists.
This page is general education, not medical advice. Talk to your own healthcare provider about your situation before starting, stopping, or changing any medication.