Table of contents
The most common side effects of obesity medication are nausea, diarrhoea, vomiting and constipation. In a pooled post hoc analysis of three STEP trials (STEP 1-3), in which semaglutide 2.4 mg (Wegovy) was compared with placebo for 68 weeks, nausea occurred in 43.9% of participants taking the medicine and 16.1% of those on placebo (1). The symptoms appear mainly in the weeks when the doctor increases the dose. They are usually mild or moderate and go away: half of the episodes of nausea resolved within 8 days, diarrhoea within 3 days and vomiting within 2 days. Constipation lasted longer: half of the episodes lasted at least 47 days. Experts advise eating smaller portions, cutting down on fatty foods and drinking regularly. If symptoms are severe, the doctor may delay increasing the dose or go back to the previous one, but you should not change the dose yourself. Severe abdominal pain spreading to the back, or vomiting that stops you keeping down fluids, means you should contact a doctor urgently.
In brief
- In trials, nausea occurred in about 25-44 out of 100 people taking semaglutide or tirzepatide. It usually starts during dose escalation, and in the STEP trials of semaglutide the largest number of people had it around week 20, a few weeks after reaching the 2.4 mg dose. After that, it eases.
- In semaglutide trials, body weight went down almost as much in people with nausea and other gastrointestinal symptoms as in people without them. Nausea is therefore not proof that the medicine is working.
- If symptoms are severe, the doctor may delay increasing the dose or go back to the previous one. Do not change the dose yourself, and never double a dose, even after a missed injection.
- Severe abdominal pain spreading to the back, pain under the right ribs with fever or jaundice, vomiting that makes it impossible to drink, and sudden loss of vision in one eye need urgent help.
How often does obesity medication cause nausea, diarrhoea and constipation?
Stomach and bowel symptoms are the most common side effects of semaglutide, tirzepatide and liraglutide, the obesity medicines given by injection. According to the official product information approved by the European Medicines Agency, nausea, diarrhoea, vomiting and constipation are very common with all three medicines, meaning they affect at least 1 in 10 people (3, 4, 5).
| Medicine and trials | Nausea | Diarrhoea | Vomiting | Constipation | Stopped treatment because of these symptoms |
|---|---|---|---|---|---|
| semaglutide 2.4 mg once a week (Wegovy), 68 weeks, post hoc analysis of the STEP 1-3 trials | 43.9% (16.1%) | 29.7% (15.9%) | 24.5% (6.3%) | 24.2% (11.1%) | 4.3% (0.7%) |
| tirzepatide 5, 10 or 15 mg once a week (Mounjaro), people without diabetes | 24.6-33.3% (9.5%) | 18.7-23.0% (7.3%) | at least 10% | at least 10% | 1.9-4.4% (0.5%) |
| liraglutide 3 mg once a day (Saxenda) | at least 10% | at least 10% | at least 10% | at least 10% | figure not reported |
The figure in brackets is the percentage of people with the symptom in the placebo group of the same trial (1, 3, 4, 5). The figures should not be compared between medicines, because they come from different trials with different groups of patients, and these symptoms also occurred at different rates in the placebo groups. For liraglutide, only the frequency category is given; in total, gastrointestinal symptoms were reported by 67.9% of participants taking this medicine (5).
In the same analysis of the STEP 1-3 trials, 72.9% of people taking semaglutide and 47.1% of people on placebo reported at least one gastrointestinal symptom. The authors of the analysis point out that such frequent symptoms on placebo may partly be due to obesity itself, which increases the risk of digestive diseases. Almost all of these symptoms were mild or moderate, and severe nausea, abdominal pain, diarrhoea or vomiting occurred in 4.1% of people on semaglutide and 0.9% on placebo (1).
What other side effects can obesity medication cause in the first months?
On semaglutide, headache and tiredness are also very common, mainly during dose escalation. Dizziness is common with all three medicines. With semaglutide, it occurs mainly in the first 4 months of treatment, and with liraglutide in the first 3 months. Liraglutide also often causes insomnia during the same period (3, 5). Redness, itching and other injection site reactions occurred in 8.0% of people on tirzepatide and 1.8% on placebo; none of them were severe (4). Semaglutide and tirzepatide speed up the heart rate by an average of 3 beats per minute (3, 4). Semaglutide and tirzepatide also commonly cause hair loss, which with semaglutide is usually mild (3, 4).
When are nausea and diarrhoea worst on obesity medication?
Nausea, vomiting and diarrhoea are most common when the dose of the medicine is going up, and they ease once the dose stops changing (3, 4, 5). This is why treatment with each of these medicines starts at a low dose, which the doctor increases gradually.
| Medicine | How the doctor increases the dose | When symptoms were most common |
|---|---|---|
| semaglutide (Wegovy), once a week | 0.25 mg in weeks 1-4, 0.5 mg in weeks 5-8, 1 mg in weeks 9-12, 1.7 mg in weeks 13-16, 2.4 mg from week 17 | during dose escalation; the largest number of people had nausea, diarrhoea and vomiting around week 20, and fewer and fewer after that |
| tirzepatide (Mounjaro), once a week | 2.5 mg for 4 weeks, then 5 mg; if the doctor considers it necessary, further increases of 2.5 mg, each after at least 4 weeks on the current dose, up to a maximum of 15 mg | during dose escalation, then less and less often; slightly more often on 10 and 15 mg than on 5 mg |
| liraglutide (Saxenda), once a day | 0.6 mg, then increases of 0.6 mg at intervals of at least a week up to 3 mg | in the first weeks of treatment; usually eased within a few days or weeks |
In the analysis of the STEP 1-3 trials, gastrointestinal symptoms began during dose escalation in the vast majority of people. After week 20, few people had them for the first time. The proportion of people with constipation stopped rising around week 10, but constipation lasted longer than the other symptoms. A small number of people had symptoms from time to time throughout treatment, although less and less often (1, 3).
Tirzepatide levels in the blood are highest 8-72 hours after the injection (4). This time is not given for Wegovy, but with semaglutide injections used for type 2 diabetes, blood levels are highest 1-3 days after the dose (6). If nausea or tiredness comes back every week on the same days after the injection, it helps to note when it starts and how long it lasts. This makes it easier for the doctor to judge whether the symptoms are easing on the current dose or whether the next increase needs to be delayed. How appetite and body weight change in the following months is covered in our article on the first months of obesity treatment with medication.
How can you ease nausea, diarrhoea and constipation during obesity treatment?
Obesity specialists advise starting with changes to eating and drinking: smaller meals, stopping eating at the first feeling of fullness, cutting down on fatty foods and drinking regularly. If this is not enough, the doctor may change the pace of dose escalation.
What should you eat if you feel sick after a weight-loss injection?
Nausea often appears in the morning or after many hours without food. Some people stop eating because of nausea, and a long gap without food makes it even worse. In a joint statement, four US scientific societies working on nutrition and obesity treatment recommend a small breakfast, then small meals every 3-4 hours and enough fluids. In the first days of treatment, avoiding fatty foods and foods high in fibre may help. Vomiting is more likely after a large meal. The authors of the statement also write that ginger or peppermint tea may help some people, and if nausea is severe, the doctor may prescribe an anti-sickness medicine for short-term use (2).
Alcohol can make nausea and heartburn worse during treatment (2). How alcohol interacts with each of these medicines is described in our article on obesity medication and alcohol. What to eat to get enough protein and avoid losing muscle when your appetite is smaller is explained in our article on what to eat on obesity medication.
How can you avoid dehydration with vomiting and diarrhoea on obesity medication?
Vomiting and diarrhoea make the body lose water, and dehydration can impair kidney function, in rare cases even leading to acute kidney failure. Older people and people with kidney disease are at greater risk (3, 4). In this situation, the patient leaflets advise drinking plenty of fluids, and with diarrhoea it may also help to avoid large and fatty meals.
Signs of dehydration may include thirst, dark, strong-smelling urine, passing urine less often, a dry mouth, tiredness and dizziness. Contact a doctor urgently if dizziness when you stand up does not go away, your urine is dark or you are passing less than usual, or you develop tiredness that is unusual for you, fast breathing or a fast heartbeat (7).
What can you do about constipation on semaglutide or tirzepatide?
Constipation lasts longer than nausea: in the analysis of the STEP 1-3 trials, half of the episodes of constipation lasted at least 47 days, and at least 35 days in the placebo group (1). It is better to deal with it straight away rather than wait for it to pass. Experts from US obesity societies advise drinking enough and gradually increasing the amount of fibre in your diet, for example from prunes and other dried fruit. Fatty foods slow stomach emptying even further and can make constipation worse. If changes to your diet are not enough, ask a doctor or pharmacist about a laxative, for example one containing fibre or macrogol (2). Severe constipation with abdominal pain, bloating and vomiting may mean a bowel obstruction and needs urgent help (3).
Why do obesity medicines cause heartburn, burping and bloating?
Medicines in this group slow stomach emptying, especially at the start of treatment, so food stays in the stomach longer and you may feel full, bloated or sick. Heartburn, burping, bloating and indigestion are common side effects of semaglutide, tirzepatide and liraglutide, meaning they affect at least 1 but fewer than 10 people in 100 (3, 4, 5). The same rules as for nausea help: smaller portions and stopping eating as soon as you feel full.
Can you lower the dose of obesity medication yourself because of side effects?
No. Your doctor decides whether to delay a dose increase, go back to the previous dose or stop treatment. If gastrointestinal symptoms are severe, the doctor may delay increasing the semaglutide dose or go back to the previous dose until the symptoms ease. The Polish obesity treatment guidelines recommend the same (3, 8).
In the analysis of the STEP 1-3 trials, the dose was reduced or treatment was temporarily interrupted because of such symptoms in 12.5% of people taking semaglutide. Three quarters of them (74%) were still taking the medicine at the end of the trial (1). With liraglutide, if the patient tolerates the medicine poorly for two consecutive weeks after a dose increase, the doctor considers ending treatment (5).
The leaflets for semaglutide and tirzepatide state that you should not stop taking the medicine without consulting a doctor, and that in people with type 2 diabetes blood glucose may rise after tirzepatide is stopped (3, 4).
This article does not replace a consultation with a doctor who knows your medical conditions and the medicines you take. In the Nasz Gabinet programme, the doctor reviews the measurements from the app every month and decides on the dose and the next e-prescription. See how obesity treatment at Nasz Gabinet works.
What should you do if you miss an injection because of nausea?
If you missed an injection because of severe or persistent nausea, contact the doctor to agree on further dosing. If you simply forgot a dose, a semaglutide injection can be taken up to 5 days after the scheduled day, and tirzepatide up to 4 days after the scheduled day. After that, the dose is skipped and the next injection is given on the usual day. A missed dose of liraglutide is taken if less than 12 hours have passed since the usual time. A missed dose is not made up for with a double dose. After several missed doses of semaglutide, the doctor considers restarting treatment at a lower dose, and with the other medicines the doctor also decides on the dose after a break (3, 4, 5).
Which symptoms on obesity medication need urgent help?
Urgent medical help is needed for symptoms that may be caused by rare but serious complications: pancreatitis, gallstones, dehydration that damages the kidneys, bowel obstruction, damage to the optic nerve or a severe allergic reaction (3, 4, 5).
| Symptom | What it may mean | What to do |
|---|---|---|
| severe, persistent pain in the upper abdomen, often spreading to the back, sometimes with vomiting | acute pancreatitis | contact a doctor straight away; if pancreatitis is suspected, the medicine is stopped, and if it is confirmed, the medicine is not restarted |
| pain under the right ribs, fever, yellowing of the skin or the whites of the eyes | gallstones or inflammation of the gallbladder | urgent medical assessment; sometimes the gallbladder needs to be removed |
| vomiting that stops you keeping fluids down, or diarrhoea you cannot keep up with by drinking, passing little urine, dizziness when standing up | dehydration, which can damage the kidneys | contact a doctor the same day |
| severe constipation with abdominal pain, bloating and vomiting | bowel obstruction | urgent medical help |
| sudden loss of vision or rapidly worsening eyesight, usually in one eye | damage to the optic nerve (NAION) with semaglutide | contact a doctor immediately and have an eye examination |
| swelling of the face, lips, tongue or throat, breathlessness, dizziness, loss of consciousness | severe allergic reaction | immediate medical help |
| a fast heartbeat or palpitations at rest that do not go away | increased heart rate | tell your doctor; with liraglutide, a sustained increase in heart rate is a reason to stop treatment |
| shaking, sweating, intense hunger, confusion in someone with diabetes treated with insulin or a sulfonylurea | low blood sugar (hypoglycaemia) | follow the doctor's instructions for low blood sugar and tell the doctor |
In semaglutide trials, acute pancreatitis was confirmed in 0.2% of people taking the medicine and in less than 0.1% of people on placebo. Gallstones were found in 1.6% of people on semaglutide and 1.1% on placebo, and inflammation of the gallbladder in 0.6% and 0.3% (3). In tirzepatide trials in people with overweight or obesity, acute gallbladder disease occurred in 2.0% of those taking the medicine and 1.6% of people on placebo (4). The risk of gallstones increases when body weight falls faster than 1.5 kg a week, whether on a diet of under 800 kcal a day, on medication or after bariatric surgery (8).
Damage to the optic nerve is very rare: large observational studies in people with type 2 diabetes show about one additional case per 10,000 person-years of semaglutide treatment, that is, per 10,000 people treated for a year (3). If you have symptoms that need immediate help, get emergency care straight away rather than waiting to reach the clinic. What to do about palpitations and low blood sugar is described in the table above.
What should you tell the anaesthetist before a procedure if you take obesity medication?
Aspiration pneumonia (lung inflammation after inhaling stomach contents) has been reported in people taking these medicines who had a general anaesthetic or deep sedation. The stomach empties more slowly on these medicines, so before an operation, or before a gastroscopy or colonoscopy under deep sedation, you need to tell the anaesthetist that you take semaglutide, tirzepatide or liraglutide (3, 4, 5). The product information does not say whether, or how many days before the procedure, the medicine should be stopped. This is decided by the anaesthetist together with the doctor managing your obesity treatment.
What are the side effects of naltrexone/bupropion (Mysimba)?
Mysimba is a tablet with two active substances, naltrexone and bupropion, which act on the centres in the brain that regulate hunger and the sense of reward, although the exact mechanism is not known. The medicine does not work like the hormone GLP-1, so its side effects and warnings differ from those of the injections.
How often does Mysimba cause nausea and constipation?
In trials, nausea occurred in 31.8% of people taking Mysimba and 6.7% of people on placebo, constipation in 18.1% and 7.2%, and vomiting in 9.9% and 2.9%. Headache is also very common, and dizziness, insomnia and dry mouth are common. People with a dry mouth more often had toothache and tooth decay. Nausea usually appeared in the first 4 weeks. Most cases resolved within 4 weeks, and almost all by week 24 of treatment. Vomiting usually lasted no longer than a week (9).
The dose is increased over 4 weeks: in the first week, one tablet in the morning; in the second, one in the morning and one in the evening; in the third, two in the morning and one in the evening; and from the fourth, two in the morning and two in the evening. A missed tablet is not made up for; you simply take the next one at the usual time. In trials, 23.8% of people on Mysimba and 11.9% on placebo stopped the medicine because of side effects (9).
When should you stop Mysimba straight away and see a doctor?
Before starting the medicine and regularly during treatment, the doctor measures your blood pressure and heart rate. A sustained rise in blood pressure or heart rate is a reason to stop the medicine, and hypertensive crisis has been reported during dose escalation. Seizures occur in about 1 in 1,000 people. After a seizure, the medicine is never taken again, and the risk is increased by, among other things, alcohol misuse, a previous head injury and certain medicines (9).
No suicide attempts were recorded in the Mysimba trials, but after the medicine was placed on the market, suicidal thoughts and behaviour were reported in people of all ages. This is why, especially at the start of treatment and after each change of dose, the patient and their family should watch for changes in mood and behaviour. If you have suicidal thoughts, low mood or unusual changes in behaviour, contact a doctor immediately or, if you cannot, go to hospital (9).
If you develop a widespread rash with blisters, peeling skin, mouth ulcers and fever, stop the medicine and see a doctor straight away. Naltrexone blocks the effect of opioid painkillers, so if you are having a planned procedure after which you may need them, tell the doctor that you take Mysimba, because it is stopped at least 3 days beforehand (9).
Frequently asked questions
Does nausea on obesity medication mean the medicine is working?
No. In a later analysis of the STEP 1-3 trials, people without gastrointestinal symptoms lost almost as much weight as people with such symptoms, and according to the official product information for Wegovy, body weight went down regardless of nausea, vomiting or diarrhoea (1, 3). In the STEP 1 trial, people who took the medicine for the full 68 weeks lost on average 15.7% of their body weight if they had no such symptoms, and 17.3% if they did. The authors of the analysis estimated that nausea, vomiting and diarrhoea accounted for less than 1 percentage point of the difference in weight loss between the medicine and placebo (1). Not having nausea is therefore not a reason to ask for the dose to be increased faster. If your body weight has not gone down for a long time, we describe the possible causes in our article on weight stalling on medication.
Do raised lipase or amylase levels in a blood test mean pancreatitis?
Not always. In tirzepatide trials in people treated for obesity, lipase activity rose on average by 34% and amylase by 23%. If there is no abdominal pain or other symptom of pancreatitis, a rise in these enzymes alone does not mean pancreatitis (3, 4). However, you should show the result to the doctor managing your treatment, and if you have severe abdominal pain, contact them straight away. We write about the tests the doctor orders before starting the medicine and during treatment in our article on tests before obesity treatment.
Can you take obesity medication if you have had pancreatitis?
Past pancreatitis is not a contraindication to semaglutide and tirzepatide, but it calls for caution. Tirzepatide has not been studied in people who have had pancreatitis (3, 4). The doctor must know about it before writing a prescription and will assess whether the benefit of treatment outweighs the risk.
Can you drive while taking obesity medication?
Semaglutide, tirzepatide and liraglutide have no or negligible effect on the ability to drive. However, all three can cause dizziness, with semaglutide and liraglutide mainly in the first months of treatment, and you then need to take care when driving (3, 4, 5). People with diabetes treated with insulin or a sulfonylurea need to watch out for low blood sugar. Mysimba can cause dizziness, drowsiness, loss of consciousness and seizures. If you have any of these symptoms, do not drive or operate machinery, and contact a doctor. After a seizure, Mysimba must not be taken again; returning to driving requires a separate medical assessment (9).
Sources
- Wharton S, Calanna S, Davies M et al. Gastrointestinal tolerability of once-weekly semaglutide 2.4 mg in adults with overweight or obesity, and the relationship between gastrointestinal adverse events and weight loss. Diabetes Obes Metab. 2022;24(1):94-105. PMID 34514682, DOI
- Mozaffarian D, Agarwal M, Aggarwal M et al. Nutritional priorities to support GLP-1 therapy for obesity: a joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. Am J Clin Nutr. 2025;122(1):344-367. PMID 40450457, DOI
- European Medicines Agency. Wegovy (semaglutyd) - charakterystyka produktu leczniczego i ulotka dla pacjenta (summary of product characteristics and patient leaflet). ema.europa.eu
- European Medicines Agency. Mounjaro (tirzepatyd) - charakterystyka produktu leczniczego i ulotka dla pacjenta (summary of product characteristics and patient leaflet). ema.europa.eu
- European Medicines Agency. Saxenda (liraglutyd) - charakterystyka produktu leczniczego i ulotka dla pacjenta (summary of product characteristics and patient leaflet). ema.europa.eu
- European Medicines Agency. Ozempic (semaglutyd) - charakterystyka produktu leczniczego (summary of product characteristics), section 5.2. ema.europa.eu
- National Health Service. Dehydration. Page last reviewed 1 May 2026. nhs.uk
- Bąk-Sosnowska M, Białkowska M, Bogdański P et al. Zalecenia kliniczne dotyczące postępowania u chorych na otyłość 2024 - stanowisko Polskiego Towarzystwa Leczenia Otyłości (Clinical guidelines for the management of patients with obesity - position statement of the Polish Society for the Treatment of Obesity). Medycyna Praktyczna, special edition, September 2024. ptlo.org.pl
- European Medicines Agency. Mysimba (naltrekson, bupropion) - charakterystyka produktu leczniczego i ulotka dla pacjenta (summary of product characteristics and patient leaflet). ema.europa.eu
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In the 6-month online programme, the doctor reviews your measurements from the app every month and decides on the next e-prescription after each review. The programme also includes video consultations with a doctor and a psychologist.
