The real numbers

From the pivotal trials, over 68 to 72 weeks. These are the proportions of people reporting the symptom at any point, not the proportion feeling it all the time.

Symptom	On treatment	On placebo
Nausea	25 to 44%	10 to 16%
Diarrhoea	19 to 30%	7 to 16%
Constipation	12 to 24%	6 to 11%
Vomiting	8 to 24%	2 to 6%

The number that matters more than any of those: in trials, roughly one person in fifteen to one in twenty stopped because of side effects. Most people get symptoms; most people get through them.

A finding worth knowing about, because it is counterintuitive. In the head-to-head trial of the two main treatments, the one that produced more weight loss also had fewer people stopping for side effects, 6.1 per cent against 8.0 per cent. More effect does not automatically mean more trouble.

When it happens

Almost entirely during dose escalation, in the first few days after each increase, usually worst 24 to 72 hours after the dose. At a stable dose most people feel close to normal, just less hungry. So the pattern is not a steady background of feeling unwell, it is a handful of difficult days each time the dose goes up, five or six times over about five months.

Which leads to the most useful single piece of advice on this page: there is no prize for climbing the ladder quickly. Staying eight weeks at a dose instead of four is a perfectly reasonable clinical decision, and it is very often the difference between a patient who continues and a patient who stops.

What to eat, from an Indian kitchen

Every guide on the internet about eating through nausea was written for a Western pantry. Bland chicken soup and dry crackers are not what is in your house. This is.

Eat this

Not this

The card to give whoever cooks

In most Indian households the person on treatment is not the person deciding what is made. Print this page, or simply pass on the two lines that matter: no deep-fried food and no cream or cashew-paste gravies for the three days after the injection, and please keep some curd, dal and rice aside. That is the whole intervention, and it works better than anything we can prescribe for nausea.

Constipation, which nobody mentions

Constipation is the side effect people are shyest about and it is the one that most often makes them stop.

Water, genuinely: aim for a visible increase, not a token glass
Fibre from food first: soaked methi seeds, isabgol at night, a fruit with the skin, vegetables at two meals
Movement, even a ten minute walk after dinner
If those do not work within a few days, tell us. An osmotic laxative is easy, safe and boring, and there is no reason to suffer through this.

The ones that are rare but serious

What	How likely	What to do
Gallstones	Cholelithiasis in about 1.6% against 0.7% on placebo. A real and dose-related effect, driven by rapid weight loss rather than by the drug directly. It happens after bariatric surgery for the same reason.	Do not lose faster than about a kilogram a week. Keep some fat in the diet, which keeps bile moving. Tell us about pain under the right ribs, especially after a fatty meal.
Pancreatitis	Large meta-analyses across more than 100,000 people do not support a class-wide risk. Event rates around 0.3 to 0.4%. Usually via gallstones rather than direct toxicity.	Severe abdominal pain that bores through to the back and does not settle: stop and go to hospital the same day.
Thyroid C-cell tumours	A boxed warning based entirely on rats. Rodent thyroid cells carry these receptors densely, human ones do not. Human data are reassuring but cannot exclude a small effect, because the cancer in question is very rare.	Absolutely not for anyone with a personal or family history of medullary thyroid carcinoma or MEN2. Routine calcitonin screening before starting is not recommended and causes more harm through false positives than it prevents.
Worsening diabetic retinopathy	Seen with one molecule in a large trial, 3.0% against 1.8%. The mechanism is almost certainly the speed of the sugar correction rather than the drug, the same effect seen when insulin is started.	If you have diabetes, have your eyes examined with dilated pupils before starting, particularly if your HbA1c is above 9. We titrate more slowly if retinopathy is present.
NAION, a form of sudden vision loss	Regulators in Europe reviewed this and classified it as very rare, up to 1 in 10,000. The product information was updated. Causation is still argued about; the labelling change is real.	Sudden loss of vision or rapidly worsening eyesight: contact us immediately. This is generally irreversible, so it is worth acting on the same day.
Low blood sugar	Not caused by these treatments alone. Caused reliably when they are combined with a sulfonylurea such as glimepiride, or with insulin.	These need reducing at the start, not after the first hypo. If you are on either, do not start weight treatment without that adjustment being made.

Two things to tell other doctors

Before any operation, endoscopy, colonoscopy or dental procedure under sedation, tell the surgeon and the anaesthetist you are on this treatment. It slows stomach emptying, which raises the risk of food coming back up under anaesthesia. Current multi-society guidance has moved away from automatically stopping the drug: for many people the safer answer is clear fluids only for 24 hours beforehand, or an ultrasound of the stomach on the day. But that decision belongs to the anaesthetist, and they can only make it if they know.

If you are a woman who could become pregnant, two things. These treatments are contraindicated in pregnancy and should be stopped roughly two months before you try to conceive. And weight loss restores ovulation, so fertility often returns within weeks in women who had assumed they could not conceive. Unplanned pregnancy on this treatment is common and entirely avoidable. Use reliable contraception and talk to us before you plan a pregnancy, not after.

Stop and seek help the same day if

Severe abdominal pain going through to your back, with or without vomiting
Vomiting that stops you keeping fluids down
Sudden loss of vision or rapidly worsening eyesight
A new lump in the front of the neck, difficulty swallowing, or a hoarse voice that persists
Yellowing of the eyes or skin
Signs of a severe allergic reaction: swelling of the face or throat, difficulty breathing

Sources: SURMOUNT-1, STEP-1 and SURMOUNT-5 published results and approved prescribing information. Cleveland Clinic Journal of Medicine review, 2025. WHO statement on NAION, 27 June 2025. Multi-society peri-operative guidance, Anesthesiology, 2024.

Side effects, and how to get through them