Semaglutide safety: what to expect, what to watch for

Step 1: What happens in the first few weeks on semaglutide

The opening weeks of treatment are when most people notice side effects most acutely. Nausea is the standout one, it affects a significant proportion of users, particularly after the first injection or after a dose step. Vomiting, loose stools, constipation, indigestion and excessive burping are also reported. Fatigue and headache appear less often but are documented. These effects are directly tied to how the medicine works: slowing the passage of food through the stomach and signalling fullness to the brain means the gut takes time to recalibrate.

For the majority of people, the intensity falls meaningfully within days to a fortnight. Eating smaller portions, avoiding high-fat or very spicy meals, and staying well hydrated all help the adjustment period. If you are finding the first few weeks harder than expected, that is worth raising with your prescriber rather than pushing through in silence, dose timing and meal habits can sometimes make a real difference. Our detailed guide to Wegovy side effects covers practical management strategies.

Injection-site reactions (redness, itching or a small lump) are another common finding, especially early on. Rotating between the abdomen, thigh and upper arm, and letting the pen reach room temperature before use, reduce their frequency. These are generally self-limiting and do not require stopping treatment.

Step 2: Side effects that warrant a call to your GP or prescriber

A smaller group of side effects sit in a middle tier: not immediately dangerous, but not something to dismiss or manage alone either. Gallbladder problems fall here. Semaglutide has been associated with gallstones and gallbladder inflammation in some users, symptoms include sharp pain in the upper right abdomen, nausea after eating fatty foods, and pain that moves to the right shoulder. These need medical review; they will not resolve by waiting.

Persistent vomiting or diarrhoea can lead to dehydration serious enough to affect kidney function, particularly in people already taking medicines that act on the kidneys (such as certain blood pressure treatments). If you cannot keep fluids down for more than 24 hours, contact a clinician. If you wear contact lenses and notice blurred vision, or have type 2 diabetes and experience unusual visual changes, seek advice promptly, rapid glucose changes can sometimes affect the retina.

Low mood, depressive symptoms or thoughts of self-harm have been investigated in connection with GLP-1 medicines. The current evidence does not establish a causal link, but the MHRA continues to monitor this closely. Anyone experiencing a significant change in mood while on semaglutide should speak to their GP or prescriber without delay. Our page on semaglutide safety concerns covers these monitored signals in more depth.

Step 3: When to seek urgent medical help

Some symptoms need same-day emergency care rather than a scheduled call. The clearest signal is severe abdominal pain, particularly pain that is persistent, comes on suddenly, and radiates through to the back, with or without vomiting. This pattern can indicate acute pancreatitis, a known but infrequent risk with GLP-1 medicines. In January 2026, the MHRA issued a formal Drug Safety Update reinforcing this warning; the advice is to stop the medicine and seek immediate medical attention if these symptoms develop. Do not wait to see whether the pain settles.

Signs of a serious allergic reaction (swelling of the face, lips or throat, difficulty breathing, rapid pulse, or a widespread rash) also require emergency services (999 or A&E). Severe dehydration with confusion, very dark urine or no urine output for many hours needs urgent assessment too.

If in doubt, contact NHS 111. It is far better to call and be reassured than to wait too long. Our broader Wegovy safety overview covers risk factors and contraindications that a prescriber weighs before treatment starts, which is part of why clinical assessment exists.

Step 4: Safety over the longer term and groups for whom semaglutide is not recommended

If you are researching semaglutide with a view to taking it for months or years, a wider picture matters. Many people starting out want to understand whether semaglutide is safe across different health circumstances, and our prescribers address that question as a routine part of every consultation. The question of long-term semaglutide safety is one our prescribers hear regularly, and the honest answer is that evidence from trials running up to 68 weeks is reassuring, with ongoing post-marketing surveillance building the picture further.

There are clear groups for whom semaglutide is not suitable. It is not recommended during pregnancy, while breastfeeding, or in the months before trying to conceive, guidance from the MHRA and NHS indicates a wash-out period before conception attempts. It is not licensed for anyone under 18. A personal or close family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia type 2 is a contraindication, as is a history of pancreatitis in some presentations. Certain conditions affecting the stomach or bowel also need prescriber discussion before treatment begins, and our guide on whether it is safe to take semaglutide with specific health conditions walks through those considerations in detail.

The NHS England guidance on weight-management injections sets out the clinical context that surrounds appropriate prescribing, including advice on contraception and surgical procedures. If semaglutide sounds right for your situation, speak to our prescribers, they discuss suitability and safety as the first part of every consultation, not an afterthought.