GLP-1 side effects: what’s common and what’s serious
Direct answer
What you need to know
Evidence strength: StrongMost side effects are digestive and often improve after the body adjusts, but symptom severity matters more than how common an effect is. Severe abdominal pain, repeated vomiting with dehydration, allergic-reaction symptoms or sudden vision change need prompt medical assessment.
Key takeaways
- Mild nausea, constipation and diarrhoea are common, especially around dose increases.
- Do not increase a dose faster than your prescribed schedule.
- Inability to keep fluids down is not a routine self-care problem.
- Emergency symptoms override every other step in this guide.
Important limitation: A webpage cannot diagnose whether a symptom is caused by the medication or distinguish every complication.
Next action: Use the decision tree below, then contact your prescriber when symptoms persist, worsen or make eating and drinking difficult.
Primary sources: FDA-approved product labels
If you believe you have an emergency, call local emergency services.
Side-effect decision tree
Use symptoms—not a guess about their cause—to choose the next step.
- Emergency symptoms?Difficulty breathing, facial or throat swelling, fainting, or severe sudden symptoms: call emergency services now.
- Red-flag symptoms?Severe or persistent abdominal pain, repeated vomiting, inability to keep fluids down, very little urine, yellow skin or eyes, or sudden vision change: seek prompt medical care.
- Mild and manageable?For short-lived nausea, fullness, diarrhoea or constipation, use the self-care guidance below and contact your prescriber if it persists, worsens, or concerns you.
This tool cannot diagnose a complication. When symptoms are severe or you are unsure, choose medical assessment rather than self-care.
Two effects that are not digestive
Almost everything written about this class concerns the gut, which leaves two common experiences oddly undocumented. Tiredness is reported at rates the labels do record, and is usually confounded with eating far less rather than caused directly by the drug. In the other direction, obstructive sleep apnoea is the one condition where a GLP-1 has an approved indication for improving it rather than a warning about causing it.
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.
Which GLP-1 has the fewest side effects?
This is one of the most-asked questions about the class, and the table above is the reason it is hard to answer from it. Those columns come from separate trials, run in different populations, over different durations, against their own placebo groups. Reading down two of them and picking the smaller number compares the trials as much as the drugs, which is the same error as ranking weight-loss results across studies that were never run against each other.
There is one place the comparison is legitimate, because the two drugs were given to people in the same study at the same time. In that head-to-head, the most common adverse events in both groups were gastrointestinal, most were mild to moderate, and most occurred during dose escalation — the finding was a similarity, not a winner. What that trial can and cannot settle is set out in the one comparison of these two that was actually run as a comparison.
The more useful question is usually not which drug has the fewest effects in a population but which effects you would find hardest to live with, since they differ in kind as well as in frequency. The per-effect rates, with the placebo arm beside each one, are laid out drug by drug in the side-effect figures with their trials attached — and the practical answer to most of them is dose escalation speed, which is a conversation with a prescriber rather than a property of the molecule.
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 is still in circulation for weeks after the last injection, 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.
The effects that are being litigated
Some of the gastrointestinal effects on this page are also the subject of federal personal-injury claims, and searching for them will put a great deal of law-firm advertising in front of you. Those pages compete on how many cases have been filed, which tells a reader nothing about their own symptoms. We wrote what the eight litigated products’ own labels actually say about those injuries instead — including the difference between a warning the FDA approved and an event somebody reported, which is the distinction the whole subject turns on, and the one claim in that litigation that appears on no label at all.
A learning path for managing side effects
Start with the decision tree above. Then choose the symptom-specific guide that matches your task; use the nutrition hub when reduced intake is becoming a barrier, and the product-identification checklist if a vial or measurement instruction is unclear.
Individual side-effect guides
- Is GLP-1 safe?What the trials established, what goes wrong, and how far past the evidence long-term use runs.
- 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.