The real risks of taking semaglutide — what the evidence says

Step 1: the first few weeks, what your body is adjusting to

Most people who experience side effects with semaglutide (Wegovy) encounter them earliest, particularly in the days after the first injection or after the dose moves up. The gut feels it first. Nausea is the most commonly reported symptom; diarrhoea, constipation, indigestion and burping follow in varying combinations. Fatigue and mild headache are also reported by a meaningful proportion of patients, especially in the first fortnight.

There is a misconception worth setting down gently: some people expect these effects to mean something is going wrong. In most cases, they signal that the medicine is working as intended, slowing gastric emptying and reducing appetite in ways the digestive system needs time to adapt to. The NHS medicines page for semaglutide describes these reactions as typically mild to moderate and transient, settling as the body adjusts.

Practical steps during this period make a real difference. Eating smaller portions, choosing lower-fat foods, staying well hydrated and sitting upright after meals all help. These are not the only strategies, your prescriber or the Patient Information Leaflet that comes with the pen will have the full picture. The side effects of taking semaglutide page covers the full list in more detail, including how frequency varies by dose.

Step 2: the risk picture beyond the gut

Beyond the digestive effects, there is a smaller set of risks that require more attention. Gallbladder problems, including gallstones, occur more often in people on weight-loss medicines than in the general population, partly because rapid weight loss itself is a known gallstone trigger. Symptoms to be aware of include pain in the upper-right abdomen, particularly after eating fatty food.

Dizziness is reported by some patients and can be tied to reduced calorie intake, changes in blood pressure or dehydration from gastrointestinal illness. Injection-site reactions (redness, itching or small lumps at the injection site) are generally mild and respond to rotating the injection location between the abdomen, thigh and upper arm.

People with a history of diabetic retinopathy should be aware that rapid glucose changes have been associated with worsening of eye complications; this is a conversation for your prescriber before treatment begins. The full Wegovy side-effect profile covers these less-common reactions with clinical context, and if you want a broader picture of what to weigh up before starting, the risks of taking Wegovy page sets out the considerations in full. Fatigue and low mood can also occur; if you notice significant changes to your mental health or mood, contact your healthcare team, tiredness and low energy has its own dedicated page if that is your primary concern.

Step 3: serious warning signs, when to get medical help

This is the section most worth reading carefully. In January 2026 the MHRA issued a Drug Safety Update specifically highlighting acute pancreatitis as a known but infrequent risk of GLP-1 medicines, including semaglutide. The warning sign is severe, persistent stomach pain that may travel through to the back, sometimes accompanied by vomiting. This is different in character from ordinary nausea, it does not ease quickly and is not relieved by eating or not eating. If you experience it, stop the injection and seek medical attention promptly; do not wait to see if it passes.

Other signs that need same-day or urgent attention include symptoms of a serious allergic reaction (swelling of the face, lips or throat, difficulty breathing, rapid heart rate), signs of severe dehydration from prolonged vomiting or diarrhoea (feeling faint, very dark urine, inability to keep fluids down), or any sudden change in vision. These are uncommon, but they are real.

The MHRA's Yellow Card scheme allows patients to report suspected side effects directly, without needing to go through a GP first. Reports matter: they contribute to ongoing safety monitoring for medicines like semaglutide, which carries a Black Triangle designation (▼) indicating it is subject to additional MHRA surveillance. You can report even if you are not certain the side effect is caused by the medicine.

Step 4: who carries a higher risk, and why the prescriber assessment matters

Semaglutide is not suitable for everyone, and the risks above are not evenly distributed. People with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 are advised against it. A history of pancreatitis, certain inflammatory bowel conditions, or specific kidney and liver problems requires careful prescriber consideration before treatment begins.

Pregnancy, breastfeeding and actively trying to conceive are all situations where semaglutide is not recommended, and effective contraception is advised during treatment; the NHS England guidance on weight-management injections covers contraception and related considerations in detail. The medicine is not licensed for anyone under 18.

This is precisely why a clinical assessment exists, not as a bureaucratic step but as the mechanism for catching the situations where risk genuinely outweighs benefit. At nume, a GPhC-registered Independent Prescriber reads every consultation personally; our clinical team reviews suitability and discusses side effects as a standard part of the process, not an afterthought. If you are considering treatment, checking your eligibility is where to start, and you can read more about the broader risks and side effects of semaglutide or the Wegovy treatment overview for wider context.