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Weight-management medicines are effective, and like any medicine they carry risks. The tables below give the reported rates next to the placebo rate from the same study, because the comparator is what makes a number honest — a symptom that occurs in 14% of people taking a dummy injection is not caused by the medicine in everyone who reports it.

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Semaglutide — Wegovy

Figures come from the pooled STEP 1–4 phase 3 programme assessed by the EMA, at the 2.4 mg dose. Acute pancreatitis was independently confirmed in 4 people (0.2%) on semaglutide 2.4 mg and 1 person (under 0.1%) on placebo in the pooled programme. Of those who stopped, most did so because of stomach and bowel symptoms.

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Tirzepatide — Mounjaro

Figures come from SURMOUNT-1, reported by dose. Acute pancreatitis was independently confirmed in 3 people on tirzepatide and 1 on placebo across the SURMOUNT-1 safety dataset reviewed by the EMA.

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Liraglutide — Saxenda

Figures come from the SCALE Obesity and Prediabetes trial at 3.0 mg daily. Acute gallstone events were reported by 2.3% on liraglutide 3.0 mg versus 0.9% on placebo across the EMA weight-management trial pool, mainly gallstones (1.5% versus 0.5%). Acute pancreatitis was independently confirmed in 7 people (0.2%) on liraglutide 3.0 mg versus 1 person (under 0.1%) on placebo in the same pool.

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Orlistat

Orlistat is not a hormone and does not act on appetite; it blocks the gut enzymes that digest fat, so roughly a third of the fat in a meal leaves the body in the stool. Its side effects follow directly from that, and they are more likely after a fatty meal. There is no placebo column here — the EU Summary of Product Characteristics records these by frequency band rather than against a comparator. Orlistat can also reduce absorption of vitamins A, D, E and K; the EU product information advises a diet rich in fruit and vegetables and says a multivitamin supplement may be considered, taken at bedtime.

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What the treatment guides say about the common ones

Very common in the first weeks. Eat smaller meals, avoid fatty or fried foods, and eat slowly. Nausea usually improves within 1–2 weeks.
Drink plenty of water (6–8 glasses a day). Include fibre-rich foods like vegetables and whole grains. Gentle movement like walking can help.
Some people feel tired in the first weeks. This usually passes. Rest when needed and keep meals light.

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Contact your doctor if

  • severe or persistent nausea, vomiting or diarrhoea prevents you from eating or drinking
  • you have severe abdominal pain that does not go away
  • you notice signs of an allergic reaction — rash, itching, swelling of the face or throat
  • any symptom worries you or feels unusual
Report a suspected side effect through your message thread even if you are not sure the treatment caused it.

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Who these medicines are not for

The consultation screens for, among other things:
  • personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 (MEN 2)
  • pregnancy, planned pregnancy in the next 2 months, or breastfeeding
  • history of pancreatitis or active pancreatic disease
  • type 1 diabetes or diabetic ketoacidosis
  • severe gastroparesis or chronic severe gastrointestinal disease
For severe, sudden or unexpected symptoms, call your local emergency number or go to the nearest emergency department. Do not wait for a reply to a message — see Safety.

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Common questions

Nausea is the most frequently reported side effect. In the trials it was reported in 38.3% of people on semaglutide 2.4 mg, in up to 33.3% on tirzepatide 10 mg, and in 40.2% on liraglutide 3.0 mg — compared with 9.5% to 14.7% on placebo. It is usually most noticeable in the first weeks and improves over time.
Yes, acute pancreatitis has been reported with GLP-1 medicines. In the pooled STEP programme, pancreatitis was confirmed in 0.2% of people on semaglutide 2.4 mg and under 0.1% on placebo. In SURMOUNT-1, 3 people on tirzepatide and 1 on placebo had confirmed pancreatitis. The consultation screens for a history of pancreatitis.
Most stomach and bowel side effects improve within 1–2 weeks as the body adjusts. Nausea, fatigue and constipation are typically worst in the first weeks and lessen with time. The dose escalation schedule is designed to help the body adapt gradually.
Contact your doctor if severe or persistent nausea, vomiting or diarrhoea prevents you from eating or drinking. For severe, sudden or unexpected symptoms, call your local emergency number first. See Safety.
Orlistat can reduce the absorption of vitamins A, D, E and K. The EU product information advises a diet rich in fruit and vegetables and says a multivitamin supplement may be considered, taken at bedtime.
Gallbladder-related events occurred in 2.5% of people on semaglutide versus 1.6% on placebo. With liraglutide, acute gallstone events occurred in 2.3% versus 0.9% on placebo. The risk is higher with significant weight loss in general, not just with these medicines.
Related: semaglutide · tirzepatide · liraglutide · orlistat · what to expect in the first month · how a consultation works · safety and eligibility

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Weight-loss consultation

Answer the weight-loss questionnaire and a registered doctor reviews it. They decide whether treatment is appropriate, which one, and at what dose — and declining is a normal outcome. See what it costs and how the review works.
This page is reference information, not medical advice and not a prescription. Whether a medicine is appropriate for you is decided by a registered doctor after reviewing your consultation.