Weight loss injection side effects: common, rare and warning signs

Why weight loss injections cause side effects

Semaglutide and tirzepatide work through gut hormones that control appetite, fullness and stomach emptying. These effects explain the most common side effects: when the stomach empties more slowly and the fullness signal is stronger, large or fatty meals quickly feel too much. Nausea, bloating, belching and heartburn follow. Because the body adapts, symptoms are strongest during escalation.

Other effects come not from the medicine itself but from rapid weight loss: gallstones, temporary hair shedding, loss of muscle mass and a thinner face. And there are rare side effects worth knowing because they need quick action. This article sets out all three groups, with figures from trials and assessments by the European Medicines Agency.

Common side effects: stomach and bowel

The best data come from the STEP 1 approval trial of semaglutide 2.4 mg with 1,961 participants. With semaglutide, 44 per cent reported nausea, 32 per cent diarrhoea, 24 per cent vomiting and 23 per cent constipation; on placebo the figures were 17, 16, 7 and 10 per cent. Most events were mild to moderate and occurred during escalation; 7 per cent stopped because of side effects, versus 3 per cent on placebo. For tirzepatide, SURMOUNT-1 shows a similar pattern, and the product information for both lists nausea and diarrhoea as very common, affecting more than 1 in 10 people.

Symptom	Semaglutide 2.4 mg (STEP 1)	Placebo (STEP 1)
Nausea	44 %	17 %
Diarrhoea	32 %	16 %
Vomiting	24 %	7 %
Constipation	23 %	10 %
Stopped because of side effects	7 %	3 %

What helps with the common symptoms

The most effective measure is slow escalation: a poorly tolerated step is kept longer or reduced. Small portions, chewing slowly, stopping at the first sign of fullness, little fat and spice in the first weeks and enough fluids spread over the day all help, as does not lying down straight after meals. For constipation, fibre, fluids and exercise help; for diarrhoea, replacing fluids matters most, because heavy fluid loss strains the kidneys. Anyone on insulin or sulfonylureas should watch for low blood sugar when appetite drops.

Rare but serious side effects

Gallstones and gallbladder inflammation are more common on weight loss injections, especially with rapid weight loss; typical are colicky pains in the upper right abdomen, sometimes with fever or yellowing skin. Pancreatitis is rare but serious, with severe, persistent upper abdominal pain often radiating to the back. In both cases treatment is paused and assessed urgently.

An insurance-data analysis in JAMA 2023 (Sodhi and colleagues) found a higher risk of pancreatitis, bowel obstruction and gastroparesis in people using GLP-1 receptor agonists for weight loss than with another weight loss medicine. In absolute terms these events are rare but belong in honest information. Because the stomach empties more slowly, the anaesthetic team should know about the treatment before surgery.

In June 2025 the EMA classified NAION, a circulation disorder of the optic nerve with sudden, usually painless loss of vision in one eye, as a very rare side effect of all semaglutide medicines, affecting up to 1 in 10,000 people. In type 2 diabetes, studies suggest roughly double the risk, in absolute terms about one extra case per 10,000 treatment years.

Mood, hair, muscle, face

After case reports, the EMA safety committee reviewed data on suicidal thoughts and self-harm and concluded in April 2024 that they do not support a causal link with GLP-1 receptor agonists. Temporary hair shedding, a thinner face with looser skin, sometimes called Ozempic face, and loss of muscle mass are consequences of rapid weight loss rather than of a particular medicine. Enough protein, strength training and not losing too fast soften these effects clearly.

Interactions and who should look more closely

Because the stomach empties more slowly, other oral medicines can work later or less well. For tirzepatide, the product information specifically warns that the contraceptive pill may be less reliable for four weeks after starting and after each dose increase. Low blood sugar is rare with weight loss injections alone but more likely with insulin or sulfonylureas, whose doses are often reduced at the start. Particular care is needed after previous pancreatitis, with gallstones, severe digestive disorders, reduced kidney function and eating disorders; the medicines are not used in pregnancy and breastfeeding.

A simple diary in the first weeks, with injection day, dose, symptoms and weight, helps decide whether to keep, lower or change a step. Suspected side effects can be reported through your doctor or pharmacy; such reports are how agencies such as the EMA detect rare risks.

The overall picture

A large cohort study using health-care data (Xie, Choi and Al-Aly, Nature Medicine 2025) compared many health outcomes on GLP-1 receptor agonists with other diabetes medicines. More common were digestive problems, low blood pressure, kidney stones and drug-related pancreatitis; less common were cardiovascular events, addiction and psychotic disorders and seizures. The balance: large benefit, real risks that need to be known and monitored. The overview is in Weight loss injections; profiles are in Wegovy and Mounjaro.

When you need urgent medical help

Call 112 for severe upper abdominal pain radiating to the back, severe abdominal pain with a bloated belly and vomiting, shortness of breath or swelling after the injection and sudden loss of vision. For ongoing vomiting or diarrhoea with dehydration, or yellowing skin, outside surgery hours call the German on-call medical service on 116 117, and pause the next dose until a doctor has assessed you.