Weight-loss drug side effects and how they are managed

What are the most common side effects of weight-loss drugs?

Wegovy (semaglutide): according to the label, in the 68-week trials nausea occurred in 43.9% of people on semaglutide (placebo 16.1%), diarrhoea in 29.7% (15.9%), vomiting in 24.5% (6.3%), constipation in 24.2% (11.1%) and abdominal pain in 19.7% (10%) [1].

Mounjaro (tirzepatide): in the weight management trial without diabetes, gastrointestinal events occurred in 55.6% to 60.8% depending on the dose and in 30.3% on placebo. Nausea occurred in 24.6% to 33.3% (9.5%) and diarrhoea in 18.7% to 23.0% (7.3%) [2].

Other common side effects listed in the Wegovy label include headache, fatigue, dizziness, heartburn, flatulence and burping [1].

Read more: Semaglutide side effects on the substance page | Tirzepatide side effects on the substance page

How long do nausea and other stomach symptoms last?

In the Wegovy trials the symptoms were mostly mild to moderate and short. The median duration was 8 days for nausea, 2 days for vomiting and 3 days for diarrhoea. Constipation lasted longer, a median of 47 days [1].

In the Mounjaro trials nausea, vomiting and diarrhoea were most frequent during dose escalation and decreased over time [2].

Treatment was stopped because of gastrointestinal events by 4.3% in the pooled Wegovy label trials (STEP 1-4) [1] and by 1.9% to 4.4% in the Mounjaro weight management trial (placebo 0.5%) [2].

What does the label say about managing side effects?

According to the package leaflets there are two things you can do yourself. Drink enough fluids, because nausea, vomiting and diarrhoea can cause dehydration, which in rare cases worsens kidney function; with Mounjaro this is stressed for older people [1, 2, 3]. If nausea or vomiting bothers you a lot, tell your doctor: according to the Wegovy leaflet you and your doctor can agree to delay the next dose increase or go back to the previous dose until symptoms improve [1]. Do not change the dose on your own.

Below are further measures from the labels that the prescriber uses.

• Slow dose escalation: the Wegovy dose is raised over 16 weeks to reduce the likelihood of stomach symptoms [1]. The Mounjaro dose is raised no more often than every four weeks [2].
• Diabetes medicines: with insulin or a sulfonylurea the risk of low blood sugar rises, and it can be lowered by reducing the dose of those medicines [1, 2].
• Oral medicines: tirzepatide slows stomach emptying, and monitoring of narrow-range medicines such as warfarin is recommended at the start of treatment [2].

Gallstones and gallbladder inflammation

In the Wegovy trials gallstones occurred in 1.6% of people on semaglutide and 1.1% on placebo, and led to gallbladder inflammation in 0.6% and 0.3% [1]. In the Mounjaro weight management trials gallbladder inflammation occurred in 0.6% and 0.2%, and acute gallbladder events were more frequent the more weight was lost [2].

In a meta-analysis of 76 randomised trials and 103,371 patients, GLP-1 drugs were linked to a higher risk of gallbladder and bile duct disease (relative risk 1.37). The relative risk was 2.29 in weight-loss trials and 1.27 in diabetes and other trials. The risk was also higher with higher doses and with longer use [4]A.

In another meta-analysis of 55 trials the risk of gallstones was 1.46 times higher, about 2 extra cases per 1,000 people treated [5]A.

Pancreatitis: symptoms and what to do

According to the Wegovy package leaflet, signs of pancreatitis may include severe, long-lasting stomach pain that may spread to the back, and then you should see a doctor immediately [1].

According to the labels, the medicine is stopped if pancreatitis is suspected and not restarted if it is confirmed. Caution is advised after previous pancreatitis. A rise in pancreatic enzymes alone does not predict pancreatitis [1, 2].

In the Wegovy trials, confirmed acute pancreatitis occurred in 0.2% of people on semaglutide and under 0.1% on placebo. In the SELECT cardiovascular trial the figures were 0.2% and 0.3% [1]. In a meta-analysis of randomised trials, GLP-1 drugs probably had little or no effect on the risk of pancreatitis [5]A.

Do weight-loss drugs take muscle too?

When weight falls, part of it is lean tissue. In a substudy of 160 SURMOUNT-1 participants, body composition was measured with DXA over 72 weeks. With tirzepatide, weight fell by 21.3%, fat mass by 33.9% and lean mass by 10.9%. About 75% of the weight lost was fat and 25% lean mass, and the proportions were the same in the placebo group [6]A.

According to the Wegovy label, in the body composition substudy of STEP 1 fat mass fell more than lean mass, and the fat lost was partly visceral fat [1].

Lean mass is not the same as muscle, and these measurements say nothing about muscle strength. Trials that combined exercise with the medicine are covered in the guide on stopping.

Read more: Stopping, weight regain and exercise

Can a weight-loss drug affect eyesight? NAION and semaglutide

On 6 June 2025 the EMA safety committee PRAC concluded that NAION (non-arteritic anterior ischaemic optic neuropathy), a condition of the optic nerve blood supply, is a very rare side effect of semaglutide, meaning it may affect up to 1 in 10,000 people. The conclusion covers Ozempic, Rybelsus and Wegovy [7].

According to population studies, semaglutide was linked to about a two-fold NAION risk in adults with type 2 diabetes, roughly one extra case per 10,000 person-years of treatment [7]. In a Danish-Norwegian registry study the incidence was 2.19 and 2.90 cases per 10,000 person-years among semaglutide users and 1.18 and 0.92 among users of the comparator drug [8]B.

If eyesight suddenly fails or worsens quickly, contact a doctor without delay. If NAION is confirmed, semaglutide is stopped [7]. In people with diabetes semaglutide can also worsen diabetic retinopathy, which is monitored closely [1].

Mood, depression and suicidal thoughts

In 2023-2024 the EMA's PRAC assessed case reports of suicidal thoughts in users of liraglutide and semaglutide. In April 2024 it concluded that the available evidence does not support a causal link between GLP-1 drugs and suicidal or self-injurious thoughts and actions, and the labels were not changed [9].

A Swedish-Danish registry study compared 124,517 people starting a GLP-1 drug with people starting another diabetes medicine. The risk of suicide death was not significantly higher, and new depression or anxiety did not differ. Most participants had diabetes [10]B.

If your mood changes in a worrying way during treatment, tell your doctor.

Other side effects: hair loss, heart rate, low blood sugar, skin sensations and the thyroid

• Hair loss: in the Wegovy trials 2.5% on semaglutide and 1.0% on placebo; 5.3% on the 7.2 mg dose. It was more frequent in those who lost at least 20% of their weight [1].
• Heart rate: in the Wegovy trials resting heart rate rose by a mean of 3 beats per minute [1].
• Low blood sugar: in people with type 2 diabetes, semaglutide and tirzepatide trials showed more low blood sugar than placebo. The risk is higher when a sulfonylurea or insulin is used at the same time [1, 2].
• Altered skin sensation according to the label: 2.1% with semaglutide 2.4 mg and 21.6% with 7.2 mg in the STEP UP trials; most recovered while treatment continued [1].
• Allergic reactions: severe allergic reactions are rare and need immediate care [1]. In the Mounjaro weight management trials hypersensitivity reactions occurred in 5.0% and 3.8% on placebo [2].
• Bowel obstruction: reported after Wegovy came on the market, frequency not known [1].
• Thyroid: in long-term animal studies semaglutide caused thyroid C-cell tumours in rats and mice, and tirzepatide in rats. According to the labels, the relevance of the semaglutide finding for humans is considered low but cannot be fully excluded, and the relevance of the tirzepatide finding for humans is unknown [1, 2].

Surgery, anaesthesia and endoscopy

Cases of stomach contents entering the lungs during general anaesthesia or deep sedation have been reported in people taking GLP-1 drugs. According to the labels, the higher risk of residual stomach content due to slower stomach emptying must be considered before such procedures [1, 2]. So mention the medicine before surgery or an endoscopy.

Pregnancy and contraception

Semaglutide is not used during pregnancy, and it is stopped at least two months before a planned pregnancy because of its long half-life [1]. Tirzepatide is stopped at least one month before a planned pregnancy, and contraception is recommended during treatment for women who could become pregnant [2]. Liraglutide is stopped if pregnancy is wanted or occurs [3].

According to the Mounjaro label, a single tirzepatide dose lowered combined contraceptive pill levels, but the change was not considered clinically relevant [2].

What users themselves report about side effects

This section is users' own accounts, not research. The writers are self-selected, the accounts are not verified, and the shares do not tell how common the side effects are [12]E, not peer-reviewed.

The data: 2,899 public posts from five English-language Reddit communities (r/Semaglutide, r/Ozempic, r/WegovyWeightLoss, r/Mounjaro and r/Zepbound) dated 14 Oct 2025 to 12 Oct 2026, a random sample from every week. 1,033 of them contained both side-effect wording and first-person wording. We read a random sample of 500 and hand-coded the 336 accounts in which the writer was using semaglutide or tirzepatide and described their own side effects or said they had none. They came from 334 people: 192 had used semaglutide (Ozempic, Wegovy, Rybelsus) and 156 tirzepatide (Mounjaro, Zepbound), and 14 had used both. Posts that looked like marketing were left out. We do not show writers' names, links, sellers or doses [12].

Check: a second coder re-coded a random tenth (50 posts) without seeing the first coder's codes. The main fields (included or not, drug, side effect or not, stopped or not) matched in 47 of 50 posts, and whether there was a side effect at all in 49. Rating how strong a side effect was proved less certain: it matched in 40 of 50, and the first coder more often rated it severe, so the severe share below may be on the high side [12].

Semaglutide, tirzepatide and liraglutide each have their own experience summary (links below). Their side-effect shares are lower than in this section because their posts are drawn from all posts about the drug, while this section's posts were chosen for side-effect wording [12].

300 of 334 people (90%) reported some side effect, and 34 (10%) said they had none or hardly any. The most common were nausea (116, 35%), stomach pain, bloating or gas (51, 15%), tiredness or low energy (48, 14%), vomiting (46, 14%) and diarrhoea (43, 13%). 192 people (57%) reported some stomach or bowel symptom [12].

Of the 298 people who described how strong a side effect was, 105 (35%) called it mild, 120 (40%) clearly bothersome and 73 (24%) severe, for example an emergency visit, vomiting with fluids not staying down, or being unable to work [12].

Of the 274 who said when it happened, 117 described side effects at the start of treatment and 79 after a dose increase, 39 said symptoms came back in the days after every injection and 22 said they began only after months. Of the 201 who said how long it lasted, 100 (50%) said symptoms passed within a week, 60 (30%) within weeks and 41 (20%) that they lasted over a month [12].

25 people (7%) said they had stopped or paused the medicine, or had decided to stop, because of side effects, and 6 switched to the other GLP-1 drug because of them. 29 (9%) described mood symptoms. According to the EMA, the evidence does not support a causal link between GLP-1 drugs and suicidal thoughts [9], but a change in mood is worth telling your doctor about [12].

What helped: few described self-care. 7 mentioned smaller or less fatty meals, 4 drinking enough, 4 fibre and 2 rest, and 14 said they turned to a doctor. We do not repeat dose changes, because those are for the doctor to decide; the measures in the label are above [12].

Single accounts described serious events whose link to the medicine cannot be judged from the account: gallstones or gallbladder removal (6 people), an emergency visit (7), a hospital stay (2), a widespread rash or hives suspected to be allergic (5), pancreatitis (1), a stomach that stopped emptying, gastroparesis (1) and suicidal thoughts (1) [12].

Why do the figures differ from the trials? In the Wegovy trials nausea occurred in 43.9%, diarrhoea in 29.7% and constipation in 24.2% [1]; in the accounts of people who had used semaglutide the shares were 36%, 13% and 7% [12]. Trials ask everyone about every symptom at regular visits, while people write on Reddit about what bothers them right now. Mild symptoms and ones already over go unmentioned, and on the other hand severe symptoms are more likely to be written about. So the shares in the accounts can be too low just as well as too high. The accounts show what side effects are like in daily life and when they come, but they cannot tell how common they are.

Short excerpts in the writers’ own words. We picked excerpts from every group: no side effects, mild ones, long-lasting ones, people who stopped and someone with a serious event.

• "I dreaded the side effects - and I've had none." (r/Zepbound, 2025-11, tirzepatide, weight now down to the normal range) [12]
• "The side effects have sometimes been rough, but nothing crazy." (r/Ozempic, 2026-02, semaglutide, tiredness in the first weeks plus nausea and diarrhoea) [12]
• "It took probably 9 or 10 weeks for the nausea to get under control" (r/WegovyWeightLoss, 2026-05, semaglutide starting phase, nausea for half of every week) [12]
• "I did not expect to functionally feel depressed every day" (r/Mounjaro, 2026-08, tirzepatide, months of low mood and lost enjoyment) [12]
• "The nausea, weakness, dizzy/ vertigo was so intense that I stopped" (r/Ozempic, 2026-09, first try of semaglutide, 2-3 weeks) [12]
• "I was so convinced I had no side effects and the gallstones left no symptoms at ALL" (r/Zepbound, 2026-07, tirzepatide, emergency surgery for gallstones and pancreatitis after about five months of use) [12]

Read more: Semaglutide: user experiences | Tirzepatide: user experiences | Liraglutide: user experiences

When to see a doctor

According to the package leaflets and the EMA, contact a doctor or emergency care straight away in these situations:

• Severe, long-lasting stomach pain that may spread to the back (a possible sign of pancreatitis) [1].
• Sudden loss of vision or rapidly worsening eyesight [1, 7].
• Difficulty breathing, swelling of the face or throat, light-headedness or fast heartbeat (a severe allergic reaction) [1].
• Vomiting or diarrhoea that stops you keeping fluids down (risk of dehydration) [1, 2].
• If you have used a product bought online "for research use" and get symptoms, Fimea advises telling healthcare staff what you used [11].

Read more: Peptide side effects in general | Risks of products bought without a prescription