Semaglutide side effects: what's common, what's rare, and when to call for help

The misconception worth clearing up first: semaglutide doesn't make everyone ill

A lot of people arrive having read that semaglutide causes weeks of relentless nausea. That reputation isn't entirely wrong, but it's not the whole story either. In the STEP 1 trial (published in the New England Journal of Medicine) the most common side effects of semaglutide were indeed digestive, but the majority were mild to moderate in severity. Many participants noticed them mainly in the first few weeks, particularly after starting or stepping up a dose, then found they quietened down considerably.

The graduated titration schedule exists precisely because of this. Beginning at a lower strength gives the gut time to adjust before moving to a maintenance dose. Not everyone feels anything noticeable at all, particularly those who are careful about portion sizes and eating slowly during the early weeks. That said, it would be misleading to suggest it's always easy, some people do find the GI effects disruptive, and that is exactly the conversation to have with a prescriber before starting.

If you want a fuller breakdown of what the UK experience looks like in practice, the Wegovy side effects page covers the injection's profile in more detail.

What the side effects of semaglutide actually are, and how they tend to behave

Nausea tops the list. It tends to arrive with the first injection or tablet and is usually at its worst for a day or two afterwards. Loose stools, constipation and indigestion follow in frequency. Some people report burping, reflux or a general feeling of fullness that lingers. Fatigue and mild headache show up in a smaller group, often in the first fortnight.

The pattern for most of these semaglutide UK side effects is similar: most noticeable early or after a dose step-up, then improving as the body adapts. Eating smaller meals, avoiding fatty or very rich food, staying well hydrated and not lying down immediately after eating all help. These aren't prescriptive instructions, your prescriber and the Patient Information Leaflet will give you guidance suited to your specific situation.

Less common effects include gallbladder problems (gallstones can occur, especially with rapid weight loss on any treatment), heart rate increases and, in people with diabetic retinopathy, changes in vision. Hair thinning is sometimes reported during significant weight loss periods, though this appears related to the caloric change rather than the medicine specifically. If you're concerned about a specific reaction, this page on more difficult semaglutide reactions addresses the less common end of the spectrum.

One area that generates questions is the eyes. Semaglutide and eye-related side effects are worth understanding separately if you have a history of diabetic eye disease.

When to get medical help, and when it's urgent

Most side effects do not require emergency care. But some do, and recognising the difference matters.

Seek urgent medical attention if you develop severe stomach pain that spreads to your back, with or without vomiting. This can be a sign of acute pancreatitis. In January 2026 the MHRA issued a Drug Safety Update specifically highlighting pancreatitis as an infrequent but serious risk with GLP-1 medicines, the safety communication is on the MHRA's Drug Safety Update index. Stop the medicine and contact a doctor or go to A&E; do not wait to see whether the pain passes.

Other signs warranting prompt contact with a healthcare professional include: symptoms of dehydration from persistent vomiting or diarrhoea (dizziness, very dark urine, inability to keep fluids down); signs of a serious allergic reaction (swelling of the face, lips or throat, difficulty breathing, severe rash); and any sudden change in vision. Low mood or thoughts of self-harm should also prompt contact with a doctor without delay.

For anything less acute (a side effect that's affecting your quality of life but isn't an emergency) the right first step is your prescriber or GP. If you want to know how long semaglutide side effects typically last before deciding whether to continue, this page looks at the duration evidence. And any suspected side effect can be reported to the MHRA via Yellow Card, whether you're still taking the medicine or not.

Starting semaglutide: what a prescriber needs to know about your health history

Semaglutide isn't suitable for everyone, and the side effect profile shifts depending on your starting health. A history of pancreatitis, certain thyroid conditions (specifically medullary thyroid carcinoma or Multiple Endocrine Neoplasia type 2), gallbladder disease, or inflammatory bowel conditions are all relevant to the prescribing decision, and people sometimes ask whether there is a cancer risk associated with Wegovy given those thyroid concerns. Pregnancy, breastfeeding and trying to conceive are contraindications; semaglutide is not licensed for anyone under 18.

This is why clinical assessment isn't a formality. At nume, a GPhC-registered prescriber reads your consultation (a real clinician, the same day you submit it) rather than an automated system. That review considers not just whether you meet the eligibility criteria on paper, but whether semaglutide's specific profile is the right fit given your history. Our weight loss treatment consultation is the starting point, and understanding the side effects from semaglutide and your individual risk are part of that conversation. You can also read about our clinical team and the standards we work to. If you'd prefer to explore the broader treatment options first, the weight loss overview sets out what's available.