GLP-1 medicines such as semaglutide (Wegovy, Ozempic) and tirzepatide (Zepbound, Mounjaro) slow stomach emptying and reduce appetite. That’s how they work, and it’s also why most of their side effects involve the stomach and gut. This guide is for people who have been prescribed one of these medicines and want to know what’s normal.
The most common side effects
- Nausea: the most common, affecting many people when they start or increase the dose.
- Diarrhoea or constipation.
- Vomiting, especially after large or fatty meals.
- Stomach pain, bloating, burping and heartburn.
- Tiredness and, for some, headaches or dizziness.
These are usually worst in the first few days after starting or moving up a dose, and settle over the following weeks. That’s why doses are increased slowly.
How to manage nausea
- Eat smaller meals and stop as soon as you feel full. The medicine makes you feel full sooner than you expect.
- Eat slowly and avoid fried, greasy and very sweet foods.
- Don’t lie down straight after eating.
- Sip water through the day. Bland foods such as crackers, toast and rice can help on bad days.
- For heartburn, see our guide to heartburn relief.
How to manage constipation
- Drink plenty of water. Eating less often means drinking less too.
- Increase fibre gradually: vegetables, fruit, beans and whole grains. See fibre for weight loss.
- Keep active. A daily walk helps the bowels move.
- If needed, ask a pharmacist about a gentle laxative. See constipation relief.
Protect your muscle
Rapid weight loss can include muscle as well as fat. Eat protein at every meal and do strength exercises twice a week. See protein for weight loss.
Warning signs: contact a doctor straight away
- Severe stomach pain that doesn’t go away, sometimes spreading to the back, with or without vomiting. This can be a sign of pancreatitis.
- Pain in the upper right of the abdomen, fever, or yellow skin or eyes: possible gallbladder problems.
- Vomiting or diarrhoea that stops you keeping fluids down, very dark urine or dizziness: dehydration can harm the kidneys.
- A lump or swelling in the neck, trouble swallowing or a hoarse voice.
- Signs of an allergic reaction: swelling of the face or throat, difficulty breathing, severe rash.
- Low mood or thoughts of self-harm: tell your doctor about any changes in mood.
- If you also take insulin or a sulfonylurea for diabetes, symptoms of low blood sugar such as shakiness, sweating and confusion.
Other things to tell your doctor or pharmacist
- Surgery or a procedure under sedation: tell the team you take a GLP-1 medicine, as a slowed stomach can raise the risk of inhaling food.
- Birth control: tirzepatide can make birth control pills less effective for 4 weeks after starting and after each dose increase. Use a backup method or a non-oral option.
- Pregnancy or planning one: these medicines are usually stopped beforehand.
Never use unofficial products
The FDA has warned about counterfeit semaglutide and unapproved versions sold online. Only use these medicines with a prescription, from a licensed pharmacy, and don’t change your dose without your doctor. Read more in weight-loss medicines explained and Ozempic vs Mounjaro.
More in our Weight Management guide: all our articles on this topic in one place.
This article is for general information only and isn’t a substitute for advice from a doctor or pharmacist. Always read the label before using any medicine.
