What are the side effects of GLP-1 medicines like Ozempic and Mounjaro?
How common each one is, when it happens, what helps, and the rare problems that need a doctor straight away.
Most people on GLP‑1 medicines like Ozempic, Wegovy and Mounjaro get stomach side effects: feeling sick, diarrhoea, vomiting or constipation. They are usually mild, come with each dose increase and fade over weeks. Serious problems are uncommon (gallstones) or rare (pancreatitis, kidney injury from dehydration), but they need urgent care.
Key facts
- How common
- 44 in 100 felt sick on semaglutide 2.4 mg, vs 17 in 100 on a dummy injection1
- Which doses
- These figures are for the weight-loss doses: semaglutide 2.4 mg (Wegovy) and tirzepatide up to 15 mg.1,3 Ozempic, the diabetes brand of semaglutide, goes up to 2 mg.9
- When
- Mostly in the first 5 months, while the dose goes up. In the semaglutide trials they were most common around week 204
- Stopped because of any side effect
- 7 in 100 on semaglutide, vs 3 in 100 on a dummy injection1
- Serious, less common
- Pancreatitis, gallbladder disease, kidney injury from dehydration, low sugar with some diabetes medicines2
- Report a side effect
- Call the government's medicine-safety helpline (PvPI) free on 1800 180 3024, on working days. How to report16
Get help now
Call 112 or go to the emergency department (casualty) if you have any of these:
- Severe stomach pain that won't go away, sometimes spreading to your back, with or without vomiting. This can be pancreatitis.2
- Vomiting or diarrhoea that won't stop, or you can't keep fluids down. Losing fluid can harm your kidneys.2
- Yellow skin or eyes, pain in your upper stomach, fever, or pale clay-coloured stools. These can be gallbladder problems.2
- Signs of a serious allergy: swelling of the face, lips, tongue or throat, trouble breathing or swallowing, a severe rash, fainting or a very fast heartbeat.2
- Very low blood sugar (more likely if you also take insulin, or sugar pills such as glimepiride or gliclazide): shaking, sweating, confusion, slurred speech or drowsiness.2
- Sudden loss of vision, usually in one eye.8,17
See a doctor soon (within days) if you notice:
Take the medicine box or leaflet with you, so the doctor knows exactly what you are on.17
On this page
- What are the common side effects?
- When do they start, and how long do they last?
- What helps with nausea and other stomach effects?
- Side effects at a glance
- What are the serious risks?
- Will I lose muscle? And what is "Ozempic face"?
- How can they affect eating and mood?
- Are fake or unregulated products a risk?
- How do I report a side effect in India?
- Questions people ask
What are the common side effects?
Stomach and gut effects. In the main semaglutide weight trial (STEP 1, about 2,000 adults over 68 weeks), 74 in 100 people had at least one, compared with 48 in 100 on a dummy injection.1
The most common are feeling sick (nausea), diarrhoea, vomiting and constipation.1 Other effects include stomach pain, heartburn (reflux), burping, bloating and tiredness.2
A dummy injection (placebo) contains no medicine. People on it get some stomach symptoms too. So the real effect of the medicine is the difference between the two numbers.
Out of every 100 people, how many felt sick (nausea) at some point:
About 2,000 adults, 68 weeks.1
So about 27 in 100 felt sick because of the medicine itself.
Weight trials summarised on the US label, up to 72 weeks.3
So about 20 in 100 felt sick because of the medicine itself.
Most of these effects are mild or moderate. In the semaglutide trials, 98 in 100 stomach side effects were mild or moderate.4 Still, 7 in 100 people stopped semaglutide because of side effects, compared with 3 in 100 on the dummy injection.1 For tirzepatide 15 mg, it was about 7 in 100, compared with 3 in 100.3
When do they start, and how long do they last?
They build up while the dose is going up, and are most common around the time you reach the full dose. Each bout usually lasts about a week or less. Constipation is the exception.4
You start on a low dose and go up in steps, usually every 4 weeks. Going up slowly lowers the risk of stomach side effects.2,3 The diagram shows what the semaglutide trials found.4
-
Month 1Weeks 1–4Starting doseStomach effects can begin.4
-
Months 2–4Weeks 5–16
-
Month 5Weeks 17–20Full dose reachedIn the semaglutide trials, nausea, diarrhoea and vomiting peaked at about week 20, then fell.4
-
Month 6 onSteady doseMost people settleStomach effects become less common. Constipation can linger. A small number keep having effects throughout.4
-
Over monthsAs weight drops
Bars are a rough picture of how common stomach effects are, not exact numbers. Based on the main semaglutide 2.4 mg weight trials (STEP 1–3), pooled, and Mounjaro's US label (sold there as Zepbound).3,4
These are medians: half of bouts were shorter, half longer.4 One more useful finding: people with stomach side effects lost about the same weight as people without them. Feeling sick isn't what makes the medicine work.4
What helps with nausea and other stomach effects?
Smaller meals, less fatty food and enough fluids help most. If a dose step is too hard, your doctor can slow the next one down.
An expert advisory from four US obesity and nutrition societies suggests these steps:5
- Eat smaller meals, more often. Nausea often comes in the morning or after a long gap without food. A small breakfast, then small meals every 3–4 hours, can help.5
- Go easy on fatty and heavy food, especially in the first days after a dose step. Large or high-fat meals can also bring on diarrhoea.5 In an Indian kitchen, that might mean fewer fried snacks like pakoras and samosas, puris, or rich, creamy gravies for a while.
- Drink enough fluids. This helps with constipation, and protects your kidneys if you're vomiting or have diarrhoea.2,5
- Get fibre from food, like vegetables, fruit and whole grains, for constipation.5
- Try ginger or peppermint tea for nausea.5
- Keep alcohol low. It can make nausea and heartburn worse.5
Slower dose steps are a doctor's call. The Wegovy label says that if a dose step isn't tolerated, the doctor can consider waiting another 4 weeks before going up.2 Some people do better on a slower schedule.5 Don't change your dose on your own.
Doctors can also prescribe anti-nausea or constipation medicines if food changes aren't enough.5
Side effects at a glance
How often each common side effect happened on the medicine and on a dummy injection, and what to do about it.
Semaglutide: the main weight trial (STEP 1), 68 weeks (nausea, diarrhoea, vomiting, constipation, injection site); other rows from the Wegovy label's pooled trials. Tirzepatide: Mounjaro's US label (sold there as Zepbound), which pools two weight trials (SURMOUNT-1 and -2), up to 72 weeks. Rounded to whole numbers.1,2,3
What are the serious risks?
They are rare, but you should know the warning signs. Most come from the drug labels' official warnings.
The numbers below are from trials, compared with a dummy injection where possible. "Per 100 patient-years" means the number of cases if 100 people took the medicine for one year.
Inflamed pancreas (pancreatitis) Uncommon: 1 in 100 to 1 in 1,000 (EU Wegovy label)
The pancreas is a gland behind your stomach. In Wegovy's weight trials, pancreatitis was confirmed in 0.2 cases per 100 patient-years, vs under 0.1 on the dummy injection.2 Put another way: if 1,000 people took it for a year, about 2 would get pancreatitis, compared with under 1 on the dummy injection. Severe cases can be fatal.2
Watch for: severe stomach pain that won't go away, sometimes reaching your back. Get help now.2
Gallstones and gallbladder disease Common: 1 in 10 to 1 in 100 (EU Wegovy label)
In the main semaglutide weight trial (STEP 1), gallbladder problems (mostly gallstones) affected 2.6% on semaglutide vs 1.2% on the dummy injection.1 That is about 26 in every 1,000 people, vs 12 in 1,000. On tirzepatide, an inflamed gallbladder affected 0.7% vs 0.2%.3 Losing weight fast raises the risk on its own. Some cases need surgery.2
Watch for: upper stomach pain, fever, yellow skin or eyes, pale stools.2
Dehydration and kidney injury Rare
Vomiting and diarrhoea can drain fluid from your body. That can damage the kidneys suddenly. Doctors call this acute kidney injury. Some people have needed dialysis.2 In tirzepatide's trials, it affected 0.5% vs 0.2% on the dummy injection.3 The risk is higher if your kidneys are already weak.2
Vomiting or diarrhoea that won't stop: drink fluids and tell your doctor straight away.2
Low blood sugar (hypoglycaemia) If you have diabetes
Low sugar is mainly a risk for people with diabetes. It is higher if you also take insulin or a sugar pill from the sulfonylurea group, such as glimepiride or gliclazide.2 In people with type 2 diabetes, low sugar happened in 6.2% on Wegovy vs 2.5% on the dummy injection.2 That is about 62 in every 1,000 people, vs 25 in 1,000. Your doctor may lower the dose of your insulin or sugar pill when you start.2
Know the signs: shaking, sweating, dizziness, hunger, fast heartbeat, confusion. Check your sugar as your doctor advises.2
Thyroid tumour warning Unknown in humans
In rats and mice, these medicines caused tumours of certain thyroid cells, called C-cells. Nobody knows yet if they do this in people.2,3 The US labels carry their strongest ("boxed") warning about it.2
The US label says not to take them if you or a family member has had medullary thyroid cancer, a rare type of thyroid cancer. The same goes if you have MEN 2 (multiple endocrine neoplasia type 2), a rare inherited condition that causes gland tumours.2 Routine blood tests or scans to check for it are of uncertain value.3
See a doctor soon for: a lump or swelling in the neck, a hoarse voice, trouble swallowing or breathing.2
Diabetic eye disease getting worse If you have diabetes
In a heart-safety trial of semaglutide in people with type 2 diabetes (SUSTAIN-6), serious eye complications happened in 3.0% on semaglutide vs 1.8% on the dummy injection.6,2 That is about 30 in every 1,000 people, vs 18 in 1,000. The extra risk was bigger in people who already had eye damage: 8.2% vs 5.2%, or about 82 vs 52 in 1,000.2 A fast drop in blood sugar can make eye disease worse for a while.2
If you have diabetic eye disease, keep up your eye checks.2
Sudden vision loss (NAION) Very rare
NAION (non-arteritic anterior ischaemic optic neuropathy) is a sudden loss of blood flow to the nerve at the back of the eye. In June 2025, Europe's medicine regulator (EMA) listed it as a "very rare" side effect of semaglutide: up to 1 in 10,000 people.8 Studies suggest about 1 extra case per 10,000 people with diabetes treated for a year.9
Sudden loss of vision or fast-worsening eyesight: contact a doctor without delay.8
Allergic reactions Serious ones are rare
Serious allergic reactions (anaphylaxis) and swelling under the skin (angioedema) have been reported.2 Milder reactions are more common: 5% on tirzepatide vs 3% on the dummy injection, mostly rash and itching.3
Swelling of face, lips or throat, or trouble breathing: get help now.2
Surgery, sedation and endoscopy Rare
These medicines slow your stomach. There are rare reports of food coming back up into the lungs (aspiration) under general anaesthesia or deep sedation, even after fasting.2 It isn't known whether stopping the medicine first lowers this risk.2
Before any surgery or procedure, tell every doctor that you take a GLP‑1 medicine.2
Blocked or very slow gut (ileus, gastroparesis) Rare
Since these medicines went on sale, there have been reports of a stalled gut (ileus), blocked bowel and severe constipation.2,3 They aren't recommended if you already have severe gastroparesis, a condition where the stomach empties very slowly.3
A 2023 study of US health records found a higher risk of gastroparesis on semaglutide than on another weight-loss medicine, bupropion–naltrexone: 9.1 vs 3.1 cases per 1,000 people a year.10 Studies like this can't prove the medicine caused it.
Less common effects on the Wegovy label
Out of every 1,000 people in the trials, how many had each one on Wegovy, against a dummy injection:2
Will I lose muscle? And what is "Ozempic face"?
Some of the weight you lose is muscle. Eating enough protein and doing strength exercise help protect it.
In the main semaglutide weight trial (STEP 1), people lost 13.6 kg on average. About 8.3 kg (62%) was fat and 5.3 kg (38%) was lean mass.5 Lean mass is everything that isn't fat, including water and organs. Muscle is about half of it, so roughly 1 kg in every 5 kg lost is muscle.5
That's similar to other ways of losing a lot of weight fast, such as surgery or very low-calorie diets.5
So roughly 1 kg in every 5 kg lost is muscle.
How to protect your muscle
- Strength training. Exercise against resistance, like weights, bands or body-weight moves, helps keep muscle. Walking alone helps less.5
- Protein at every meal. Experts have proposed 1.2–1.6 g of protein per kg of body weight a day while losing weight, above the usual 0.8 g. For a 70 kg person, that is about 85–110 g a day. The right target for each person isn't settled.5 Indian options include dal, paneer, curd, eggs, chicken and fish. Guide 8 has more on eating.
"Ozempic face"
This isn't a medical diagnosis. It's a popular name for a face that looks thinner, older or saggier after quick weight loss.11 A 2026 review linked it to losing fat from the face, plus thinner and less stretchy skin.11

- Less fat under the skinThe face loses volume and looks thinner.11
- Thinner skinWith less collagen and elastin, the fibres that keep skin firm.11
- Less stretchSkin doesn't shrink back, so loose folds form and the face can look older.11
Older age, losing weight quickly, not drinking enough and too little protein made it worse. The review suggests slower weight loss, good nutrition and fluids.11
Hair loss
Hair thinning was reported in about 3% on Wegovy vs 1% on the dummy injection.2 On tirzepatide, it was 7% of women and under 1% of men.3 The labels link it to the weight loss. No one on tirzepatide in the trials stopped because of it.3 Heavy hair loss can also be a sign of low nutrients, so mention it to your doctor.5
How can they affect eating and mood?
Many people think about food less. Some also enjoy food less. If you have had an eating disorder, these medicines need extra care.
Eating disorders. The effect of these medicines on anorexia, bulimia or binge eating isn't well known. They could make symptoms better or worse.5 Experts advise screening before starting, and a specialist if you have a history of an eating disorder.5
Suicidal thoughts. Regulators have reviewed this. In April 2024, Europe's regulator found that the evidence does not support a link between these medicines and suicidal thoughts or self-harm.7 In early 2026, the US removed this warning from the Wegovy and Zepbound labels (Zepbound is the US name for Mounjaro).2,3
That doesn't mean mood never changes. Experts still advise checking for depression before starting.5 If your mood drops or you have thoughts of harming yourself, talk to a doctor. If you are in danger, call 112.
Are fake or unregulated products a risk?
Yes. Fake pens and badly made products can contain the wrong dose, the wrong medicine, or nothing at all.
In June 2024, the World Health Organization (WHO) warned about fake Ozempic found in Brazil, the UK and the US. It said reports of fake semaglutide have risen in all regions since 2022.12 Some fakes may contain hidden insulin.12
In the US, the FDA has had reports of people hospitalised after dosing mistakes with unapproved semaglutide. Some patients measured the wrong dose themselves.13 If you are given a vial and a syringe, Guide 6 explains how to check the dose.
WHO's advice: get these medicines with a prescription from a licensed doctor, and avoid unfamiliar or unverified sellers, including online.12 Guide 6 explains who may prescribe in India.
How do I report a side effect in India?
Tell your doctor first. You can also report it yourself to the Pharmacovigilance Programme of India (PvPI), the government's medicine-safety system.
- Call the PvPI helpline: 1800 180 3024. It's toll-free, on working days from 9:00 am to 5:30 pm, for doctors and the public.16
- Use the "ADR PvPI" app for Android phones.16
- Fill in the patient form ("Medicines side effect reporting form") on the PvPI website. It's available in Hindi and 9 other Indian languages.16
Before you report, note the brand, strength and batch number from the box.
Questions people ask
Before you start
Do GLP-1 medicines cause thyroid cancer?
They caused thyroid tumours in rodents. It's not known whether they do in people.2 The US label says people with a personal or family history of medullary thyroid cancer, or MEN 2, shouldn't take them; tell your doctor if this applies.2
Does Mounjaro cause fewer side effects than Ozempic?
You can't tell from the numbers on this page. They come from separate trials with different people and doses. Both cause the same kinds of stomach effects, mostly during dose steps.2,3
Is it safe if I'm planning a pregnancy or take the pill?
The Wegovy label says to stop at least 2 months before a planned pregnancy.2 Tirzepatide can make the contraceptive pill less reliable. Its label advises another method, or adding condoms, for 4 weeks after starting and after each dose step.3 Guide 4 has more.
Once you're taking it
Will the side effects go away?
For most people, yes. In the semaglutide trials, stomach effects peaked around week 20 and fell after that. A small number had them throughout.4
Do I lose more weight if I feel sick?
No. People with and without stomach side effects lost about the same weight on semaglutide.4
I missed doses. Can I restart at my old dose?
Not always. The Wegovy label says: if you miss one dose and your next dose is more than 2 days away, take it as soon as you can. If it is less than 2 days away, skip it and take the next one on your usual day. If you miss 2 or more weekly doses in a row, restart at a lower dose and step up again. This lowers the risk of stomach side effects.2 Ask your doctor which dose to restart on. Guide 8 covers missed doses in full.
I have surgery or an endoscopy coming up. What should I do?
Tell the surgeon, the anaesthetist and the doctor doing the procedure that you take a GLP‑1 medicine. They will decide what to do.2
Before you move on
- Stomach side effects are common, usually mild, and worst while the dose goes up.
- Small meals, less fatty food and enough fluids help. Slower dose steps are your doctor's call.
- You know the "get help now" signs: severe stomach pain, vomiting you can't stop, yellow skin, allergy, very low sugar.
- The US label rules them out with a personal or family history of medullary thyroid cancer, or MEN 2.
- Protein and strength training help protect muscle.
- You can report a side effect to PvPI on 1800 180 3024.
Sources
- Wilding JPH et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). NEJM 2021;384:989–1002. doi:10.1056/NEJMoa2032183
- US FDA. Wegovy (semaglutide) prescribing information and Medication Guide, revised June 2026 (US label). DailyMed
- US FDA. Zepbound (tirzepatide; sold in India as Mounjaro) prescribing information, revised August 2026 (US label). FDA PDF
- Wharton S et al. Gastrointestinal tolerability of once-weekly semaglutide 2.4 mg in adults with overweight or obesity (STEP 1–4). Diabetes, Obesity and Metabolism 2022;24:94–105. doi:10.1111/dom.14551
- Mozaffarian D et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint advisory (ACLM, ASN, OMA, The Obesity Society). American Journal of Clinical Nutrition 2025;122:344–367. Declared interests: 4 of 18 authors declare advisory or speaker roles with Novo Nordisk and 5 with Eli Lilly (paper's own declaration). doi:10.1016/j.ajcnut.2025.04.023
- Marso SP et al. Semaglutide and cardiovascular outcomes in patients with type 2 diabetes (SUSTAIN-6). NEJM 2016;375:1834–44. doi:10.1056/NEJMoa1607141
- European Medicines Agency. PRAC meeting highlights, 8–11 April 2024 (GLP-1 receptor agonists and suicidal thoughts). 12 April 2024. EMA
- European Medicines Agency. PRAC meeting highlights, 2–5 June 2025 (semaglutide and NAION). 6 June 2025. EMA
- European Medicines Agency. Ozempic: summary of product characteristics (EU label), section 4.8. EMA PDF
- Sodhi M et al. Risk of gastrointestinal adverse events associated with GLP-1 receptor agonists for weight loss. JAMA 2023;330:1795–7. doi:10.1001/jama.2023.19574
- Barone M et al. Effects of GLP-1 receptor agonists on skin quality: a comprehensive literature review. Aesthetic Plastic Surgery 2026;50:5403–7. doi:10.1007/s00266-026-05820-4
- World Health Organization. WHO issues warning on falsified medicines used for diabetes treatment and weight loss (Medical Product Alert 2/2024). 20 June 2024. WHO
- US FDA. FDA's concerns with unapproved GLP-1 drugs used for weight loss. Page dated 1 October 2026. FDA
- Medical Dialogues. Recalls by Dr Reddy's, Torrent put spotlight on quality standards in India's semaglutide market. 10 July 2026. Link
- Outlook India. Inspections, surveillance to continue against unauthorised sale, promotion of GLP-1 weight-loss drugs: Govt. 1 April 2026. Link
- Indian Pharmacopoeia Commission. Pharmacovigilance Programme of India: ADR reporting (helpline, app and patient forms). IPC
- NHS. Semaglutide: side effects. Page last reviewed 15 May 2026. NHS
- Press Information Bureau (PIB Research). GLP-1 drugs: use, risks, and regulation (backgrounder). 1 April 2026. PIB PDF
- CDSCO. Recommendations of the Subject Expert Committee (Endocrinology & Metabolism): 20–21 April 2023, item 1 (tirzepatide), CDSCO PDF (archived copy); 13–14 February 2024, items 3–4 (semaglutide), CDSCO PDF (archived copy).
Last checked 5 Oct 2026 · Next review due Jan 2027 · Report an error
How this guide was made. Written by the GLP Guidebook team from the sources above: drug labels, published trials, regulator notices and named news reports. Label figures come from the US and European labels, which are published in full and easy to check. It's general information, not medical advice. Talk to a qualified doctor before starting, stopping or changing any medicine. How we write