Side effects are one of the biggest concerns for anyone considering GLP-1 weight loss medication — and understandably so. The good news is that most side effects are manageable and tend to improve over time. The key is knowing what to expect, when to be concerned, and how to minimise discomfort.
Why GLP-1 medications cause side effects
The most common side effects are gastrointestinal — and they're a direct consequence of how the medication works. GLP-1 medications slow gastric emptying (how quickly food leaves your stomach), which is part of how they reduce appetite and help you eat less. But a stomach that empties more slowly can also cause nausea, bloating, and changes in bowel habits, particularly when your body is still adjusting.1
This is why all GLP-1 medications use gradual dose titration (starting low and increasing slowly) — it gives your digestive system time to adapt.
The most common side effects
These affect a substantial proportion of users but are generally mild to moderate and temporary:12
Nausea is the single most reported side effect. It affects roughly 40–44% of Wegovy users and 20–25% of Mounjaro users in clinical trials. It's typically worst during the first 1–2 weeks at a new dose and gradually subsides. Eating smaller, more frequent meals and avoiding very rich or fatty foods helps significantly.
Diarrhoea affects approximately 15–30% of users depending on the medication. Staying well hydrated is important, particularly if diarrhoea is persistent. Oral rehydration solutions can help if fluid loss is significant.
Constipation affects 10–15% of users — the opposite problem to diarrhoea, but caused by the same mechanism (slowed gut motility). Adequate fibre intake and hydration usually help. Speak to your pharmacist if it persists.
Vomiting, stomach pain, and bloating are also common, particularly during dose escalation. These usually improve within a few weeks at each dose level.
Other common effects include headache, fatigue, dizziness, injection site reactions (redness or itching at the injection site, affecting ~4% of Mounjaro users), decreased appetite (which is actually the intended effect), and hair loss (reported by some users, likely related to rapid weight loss rather than the medication itself).
Practical tips for managing side effects
- Eat smaller meals — your stomach is emptying more slowly, so smaller portions reduce nausea and bloating
- Avoid high-fat and very rich foods — these are harder to digest and more likely to trigger nausea
- Stay hydrated — water, herbal teas, and clear broths help with both nausea and constipation
- Eat slowly — rushing meals when your digestion is slower increases discomfort
- Don't skip the titration — the gradual dose increase exists for a reason. Don't rush to the maximum dose
- Time your injection — some people find that injecting in the evening reduces daytime nausea
- Prioritise protein — when eating less overall, protein helps maintain muscle mass and keeps you feeling satisfied
Serious but rare side effects
These affect a small minority of users but require awareness and sometimes medical attention:
Pancreatitis (inflammation of the pancreas) affects approximately 1 in 100 users. Symptoms include severe, persistent stomach pain that may radiate to the back, often accompanied by nausea and vomiting. If you experience these symptoms, stop taking the medication and seek medical help immediately. For more detail, see our pancreatitis risk guide.12
Gallbladder problems (gallstones, cholecystitis) can occur, likely related to rapid weight loss. Symptoms include sudden pain in the upper right abdomen, sometimes radiating to the shoulder. Seek medical advice if you experience this.
Acute kidney injury can occur in rare cases, usually as a result of severe dehydration from prolonged vomiting or diarrhoea. This is why staying hydrated is so important, and why you should contact your prescriber if you can't keep fluids down.
Severe allergic reactions (anaphylaxis) are very rare but possible with any injectable medication. Symptoms include swelling of the face, tongue, or throat, difficulty breathing, and rapid heartbeat. Call 999 immediately.
Medication-specific concerns
Mounjaro and contraception: Tirzepatide may reduce the effectiveness of oral contraceptive pills and oral HRT because it slows gastric emptying, potentially affecting absorption. The MHRA recommends using additional barrier contraception (such as condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This warning does not apply to non-oral contraception (patches, implants, IUDs, injections). For full details, see our Mounjaro and contraception guide.2
Wegovy 7.2mg and dysaesthesia: The higher 7.2mg dose of Wegovy has been associated with dysaesthesia — a tingling, pins-and-needles, or buzzing sensation on the skin. This is not nerve damage and usually improves over time, even while continuing treatment.1
Thyroid tumour warning: In the US, semaglutide (Wegovy/Ozempic) carries a boxed warning about thyroid C-cell tumours based on findings in rodent studies. However, human studies have not confirmed this risk, and the warning is not applied in the UK or EU.1
When to contact your doctor
Contact your prescriber, GP, or call NHS 111 if you experience:
- Severe, persistent stomach pain (especially radiating to the back)
- Persistent vomiting that prevents you keeping fluids down
- Signs of an allergic reaction (swelling, breathing difficulty)
- Yellowing of skin or eyes (jaundice)
- Significant changes in vision
- Signs of severe dehydration (very dark urine, dizziness, confusion)
For a medication-specific breakdown with percentages, use our side effects viewer.
Related articles
SafetyMounjaro and the Contraceptive Pill
MHRA guidance on oral contraceptives and HRT.
SafetyPancreatitis Risk with GLP-1 Medications
What the data shows and when to seek help.
SafetyGLP-1 and Muscle Loss
How to protect lean mass during treatment.