Key points
- In the main trial, about 3 in 10 people on tirzepatide had nausea, compared with 1 in 10 on placebo.
- About 3 in 4 people on semaglutide 2.4 mg had a stomach side effect, compared with about half on placebo.
- Most stomach side effects happen during dose increases and ease with time.
- Severe stomach pain, vomiting you can't stop, or signs of an allergic reaction need urgent care.
How common are side effects?
Stomach side effects are the most common by far. The tables below are from the main weight-loss trials, comparing people on the medicine with people on a dummy injection (placebo).
Tirzepatide (Mounjaro), SURMOUNT-1 trial
| Side effect | 5 mg | 10 mg | 15 mg | Placebo |
|---|---|---|---|---|
| Nausea | 25% | 33% | 31% | 10% |
| Diarrhoea | 19% | 21% | 23% | 7% |
| Constipation | 17% | 17% | 12% | 6% |
| Vomiting | 8% | 11% | 12% | 2% |
Between 2% and 4% of people on tirzepatide stopped treatment because of stomach side effects, compared with under 1% on placebo (SURMOUNT-1 and SURMOUNT-2 combined).
Semaglutide 2.4 mg (Wegovy), STEP 1 trial
About 3 in 4 people on semaglutide (74%) had a stomach side effect, compared with about half on placebo (48%). Most were mild to moderate and passed. 7% stopped treatment because of side effects, compared with 3% on placebo.
Common side effects, and what helps
Nausea
The most common side effect, usually worst in the days after an injection and after each dose increase. Eat smaller meals, eat slowly and stop at the first sign of fullness. Oily, fried and very spicy food makes it worse. Don't lie down straight after eating.
Constipation
Drink plenty of water, add fibre gradually (vegetables, fruit, whole dals) and keep moving. Ask your doctor before taking a laxative.
Diarrhoea
Drink extra fluids, with oral rehydration salts if it's frequent, and keep meals plain. Tell your doctor if it doesn't settle or you feel dizzy or very thirsty.
Vomiting
Sip fluids little and often. If you can't keep fluids down, contact your doctor the same day, because dehydration can affect the kidneys.
Indigestion, burping and bloating
Smaller meals, fewer fizzy drinks and not eating late at night usually help.
Tiredness and very low appetite
Eating much less can leave you tired. Don't skip meals entirely: keep protein at every meal, even when portions are small. Our dietitian can help you plan this.
Injection-site reactions
Mild redness or itching where you inject. Change the spot each week.
Changes that come with weight loss
Hair thinning
Some people notice hair shedding a few months into fast weight loss. It is usually temporary. Your doctor may check iron, thyroid and protein intake, and our dermatologist can help if it persists.
Muscle loss
Part of any weight lost is muscle. Enough protein and regular strength training help protect it, and InBody scans show whether you are losing fat or muscle. Read how to protect muscle.
Changes to your face
Losing weight quickly can make the face look thinner, sometimes called "Ozempic face". It's a result of fat loss, not a separate condition.
Serious but less common risks
These are rare. They are the reason these medicines are only prescribed after a doctor's assessment.
- Pancreatitis: severe stomach pain that won't go away, sometimes spreading to the back, with or without vomiting.
- Gallbladder problems, including gallstones: pain in the upper right of the stomach, fever, or yellowing of the skin or eyes.
- Low blood sugar in people who also take insulin or sulfonylureas for diabetes: shakiness, sweating, confusion.
- Dehydration and kidney problems from severe vomiting or diarrhoea.
- Serious allergic reactions: swelling of the face, lips, tongue or throat, or difficulty breathing.
- Thyroid tumours were seen in rodent studies. It is not known whether this happens in people, so anyone with a personal or family history of medullary thyroid cancer or MEN2 should not take these medicines. Tell your doctor about a lump in the neck, hoarseness or trouble swallowing.
- Diabetic eye disease: people with diabetes and eye problems should tell their doctor about any change in vision.
- Mood changes: tell your doctor if you notice low mood or unusual thoughts.
- The contraceptive pill (tirzepatide): Tirzepatide can make the contraceptive pill less reliable. If you use the pill, switch to a non-pill method or add a barrier method such as condoms for 4 weeks after starting and for 4 weeks after each dose increase.
Reporting side effects
Tell your doctor about any side effect. You can also report it to the Pharmacovigilance Programme of India (PvPI), which monitors medicine safety in India.
When to get help urgently
How Lean Life manages side effects
Your doctor starts you on the lowest dose and only increases it when you are coping well. Advisors check in between visits, and anything medical goes to a doctor. If side effects are hard to live with, the doctor can keep you on a dose for longer or bring it down.
Questions
Do the side effects go away?
For most people, stomach side effects ease within a few weeks at each dose. In the tirzepatide trials, most nausea, vomiting and diarrhoea happened while the dose was going up.
Do higher doses cause more side effects?
In SURMOUNT-1, nausea, diarrhoea and vomiting were somewhat more common at higher doses. That is why doses go up slowly, and why not everyone needs the top dose.
Are side effects different with generic semaglutide?
Generic semaglutide is the same medicine, so the same side effects are expected. Make sure the product you use is approved in India and sourced from a licensed pharmacy.
Should I stop the injections if I feel unwell?
Don't stop or change your dose without talking to your doctor, except in the urgent situations above, where you should seek care straight away.
Can I drink alcohol on these medicines?
Alcohol can make nausea worse and, for people on diabetes medicines, raise the risk of low blood sugar. Ask your doctor what's sensible for you.
References
- Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 2022
- Lilly Medical. Gastrointestinal adverse events in tirzepatide weight-management studies
- Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). N Engl J Med 2021
- Mounjaro summary of product characteristics, European Medicines Agency
- Wegovy summary of product characteristics, European Medicines Agency
- Pharmacovigilance Programme of India, Indian Pharmacopoeia Commission
