Known Side Effects of Wegovy: What the Evidence Shows

What the trial data actually recorded

The STEP 1 trial (68 weeks, 1,961 adults with obesity, published in the New England Journal of Medicine) gives the most reliable snapshot of Wegovy's side-effect profile at the 2.4mg maintenance dose. Gastrointestinal (GI) effects dominated. Nausea affected roughly 44% of participants on semaglutide, compared with around 16% on placebo. Diarrhoea, vomiting and constipation each appeared in more than 20% of the active-treatment group. Crucially, the vast majority of these events were mild or moderate in severity, and most resolved without people stopping treatment.

What the numbers also showed: GI effects clustered around dose escalation points. The titration schedule (starting at 0.25mg and stepping up every four weeks toward the 2.4mg maintenance dose) is designed specifically to give the gut time to adapt. Side effects are not random; they follow a recognisable pattern. Knowing that pattern makes them easier to manage. For a full breakdown of all reported effects across the escalation schedule, the NHS patient information for semaglutide lists them by frequency category and is updated in line with the current Summary of Product Characteristics.

Headache, fatigue and dizziness were also reported more often on semaglutide than placebo, less discussed, but worth knowing about before you start. Injection-site reactions (redness, itching, bruising at the abdomen, thigh or upper arm) were reported too, though generally minor. If you want to understand the full range of what has been recorded, our dedicated page on Wegovy side effects covers each category in detail. Understanding that predictability is useful. It means you can plan around the first few weeks rather than be caught off-guard by them.

Less common effects that still deserve attention

Beyond the GI cluster, a smaller group of effects appears in Wegovy's safety data with lower frequency but greater clinical significance. Gallbladder problems (including gallstones and inflammation of the gallbladder (cholecystitis)) are listed as uncommon in the SmPC and were observed in STEP programme data. Rapid weight loss itself can increase gallstone risk, so the medicine is not the only contributing factor; still, right-sided or upper-abdominal pain that does not resolve should not be dismissed.

Heart rate increases of around 1–4 beats per minute have been reported on semaglutide. For most people this is clinically insignificant, but it is one reason a full cardiovascular history forms part of any responsible prescribing assessment. Wegovy's broader clinical picture includes a cardiovascular benefit in eligible adults, a useful counterpoint to the heart-rate finding, and one your prescriber can contextualise for your individual situation.

Hair thinning is another effect that comes up repeatedly in patient experience. Telogen effluvium (a shedding pattern triggered by rapid weight loss rather than by the drug's direct mechanism) is the most likely explanation. It tends to be temporary. If you want a careful look at how that resolves over time, our page on whether Wegovy hair loss is permanent works through the evidence. There is also a class of rarer events (pancreatitis, acute kidney injury from severe dehydration, and hypersensitivity reactions) that the next section addresses directly, because these are the ones that need to be taken seriously rather than managed at home.

When to get medical help without delay

In January 2026, the MHRA issued a Drug Safety Update reinforcing that acute pancreatitis is a known, if infrequent, risk associated with GLP-1 medicines including semaglutide. The signal to act: severe stomach pain that builds gradually, does not respond to ordinary painkillers, and may radiate through to the back, with or without vomiting. This symptom pattern warrants same-day emergency assessment, not a watchful wait. Stop the injection and seek help. You can review the MHRA's guidance and report a suspected side effect at any time through the Yellow Card scheme.

Other situations that need prompt medical attention include: signs of a serious allergic reaction (swelling of the face, lips or throat; difficulty breathing; widespread rash); severe or persistent vomiting or diarrhoea leading to signs of dehydration such as very dark urine or inability to keep fluids down; and any sudden or significant change in vision, which is relevant for people with pre-existing diabetic retinopathy.

It is also worth knowing that if you are taking Wegovy alongside oral contraceptives, tirzepatide's absorption interaction (the four-week non-oral backup rule) does not apply to semaglutide in the same way, but the NHS England guidance on weight-management injections still advises discussing contraception with your prescriber before starting. These are exactly the conversations that happen during a nume consultation (sorry, during a nume consultation) so nothing you later wish you had asked about gets missed. Our prescribers are also available through aftercare support seven days a week once you are on treatment.

Managing side effects in practice

The practical question most people ask is not whether side effects happen but what to actually do about them day-to-day. For nausea, which tends to peak in the first one to two days after each injection, timing the dose and eating smaller, lower-fat meals around it helps considerably. Some people find that injecting before bed means the worst of the nausea passes while they are asleep. Staying well-hydrated matters more than usual, especially during the early weeks, the GI effects can catch people out, particularly mid-week after a Monday injection, and if you are already stretched thin it is easy to forget to drink enough.

Constipation is common enough that it is worth getting ahead of it: adequate fibre, movement and fluids reduce its severity. Diarrhoea at the other end of the scale generally settles on its own, but if it is severe or lasts more than a day or two, contact your prescriber. For a more complete practical breakdown of managing these effects over time, the side effects of taking Wegovy in practice goes into more detail. If you want to understand what each recorded reaction actually involves before you begin, our guide to the side effect of Wegovy works through them one by one. The less common but more serious effects are covered in their own dedicated resource if you want to read more before your consultation.

Wegovy is a prescription-only medicine, and that status exists for good reason. The prescribing assessment at nume is a clinical conversation, not a checkbox. Our prescribers discuss your health history, any medicines you already take, and the specific side effects most relevant to your situation. You can explore what that consultation involves, and whether this treatment might suit you, by visiting our treatment overview.