Adverse Reactions to Wegovy: What the Evidence Shows and When to Seek Help

What the trial data actually recorded about Wegovy side effects

The STEP 1 trial enrolled over 1,900 adults and followed them for 68 weeks. Across that period, adverse reactions to Wegovy were common but overwhelmingly mild or moderate in severity. Nausea was the most frequently reported symptom, affecting a significant proportion of participants, followed by diarrhoea, vomiting and constipation. The NHS medicines page for semaglutide lists these GI effects alongside other reported reactions including headache, dizziness, fatigue and injection-site redness or bruising.

Crucially, the timing matters. Reactions were most pronounced after starting treatment and after each dose increase, which is exactly why the prescribing schedule steps up gradually over several months. Many participants found that nausea, in particular, reduced substantially within a week or two of settling onto a given dose. That said, individual experience varies, some people sail through; others find the adjustment harder. Neither response is unusual.

For a fuller picture of the full Wegovy side-effect profile, including less common reports from post-marketing surveillance, it is worth reading through the patient information leaflet that comes with every pen, which lists reactions by frequency tier.

When an adverse reaction to Wegovy needs urgent medical attention

Most reactions are unpleasant rather than dangerous. A small number are not. Knowing the difference is the practical thing this page is here to help with.

Call 999 or go to A&E if you experience: swelling of the face, lips, tongue or throat; difficulty swallowing or breathing; a rapidly spreading rash or hives; or a sudden drop in blood pressure causing dizziness and collapse. These may indicate a serious allergic reaction. For more detail on skin and immune responses specifically, the guide to Wegovy skin reactions covers what local injection-site reactions look like versus signs of a wider allergic response.

Get urgent medical help (same day, not at your next appointment) if you have: severe, persistent abdominal pain, especially pain that radiates through to your back, with or without vomiting. In January 2026, the MHRA's Drug Safety Update for GLP-1 medicines identified acute pancreatitis as an infrequent but serious risk. It is not common, but it can be serious, and the symptom pattern above is the one to act on quickly. Gallbladder problems, including gallstones, are also more frequently reported in people on GLP-1 medicines; upper-right abdominal pain, particularly after eating, warrants a conversation with a doctor.

Sustained vomiting or diarrhoea that stops you keeping fluids down can lead to dehydration and put strain on your kidneys. If you cannot drink without vomiting for more than 24 hours, that is a reason to contact a healthcare professional.

Reactions that are uncomfortable but self-limiting, and what helps

Nausea is real and it can be genuinely disruptive, but there are practical things that reduce it. Eating smaller portions, slowing down at mealtimes, avoiding very fatty or spicy food, and staying upright after eating all make a difference for most people. One habit worth trying: before your injection day, check in briefly with how you've been eating that week, heavy meals the day before a dose tend to amplify nausea the following day.

Constipation affects a meaningful number of people on Wegovy, partly because the medicine slows gastric transit. Increasing fluid intake and dietary fibre usually helps; the guide to managing constipation on Wegovy has practical detail. Diarrhoea is the flip side and generally settles without intervention, though it is worth monitoring if it is prolonged.

Headache, fatigue and dizziness are reported, particularly in the early weeks. These often reflect reduced calorie intake as much as the medicine itself, so eating enough protein and maintaining hydration matter. Injection-site reactions (a small bruise, a patch of redness or mild itching) are common and almost always resolve within a few days. Rotating injection sites between your abdomen, thigh and upper arm reduces local build-up.

If you are finding the side-effect burden significant, the information on difficult semaglutide reactions covers options including dose pausing, which should always be discussed with a prescriber rather than decided alone.

Reporting adverse reactions and getting ongoing support

Wegovy carries a Black Triangle (▼) designation in the UK, meaning the MHRA actively collects additional safety data while it is in wider use. If you experience any adverse reaction you think is worth flagging (however minor) reporting it through the Yellow Card scheme contributes to the real-world evidence base that shapes future prescribing guidance. Patients, carers and healthcare professionals can all submit reports.

For context on rarer effects that have emerged in post-marketing reports, the overview of rare Wegovy side effects is worth reading before starting treatment so that nothing catches you completely off-guard.

At nume, our prescribers discuss the full adverse-reaction picture as part of every consultation, and our aftercare team is available seven days a week if something changes after you start. If you want to talk through whether Wegovy is suitable for you, speak to our prescribers through a free consultation, no waiting list, no referral needed.