The Bad Side Effects of Wegovy — and What the Evidence Actually Shows

What the trial data tells us about how common bad Wegovy side effects really are

The STEP 1 trial (68 weeks, nearly 2,000 adults with obesity or overweight) is the bedrock of what we know about Wegovy's side-effect profile. Roughly three-quarters of participants on semaglutide 2.4 mg reported at least one gastrointestinal side effect during the study, compared with around a third of those taking placebo. Nausea was the most common single symptom, followed by diarrhoea, vomiting and constipation.

Those numbers sound high, but the context matters. The large majority of GI events were classed as mild or moderate in severity, and most resolved without stopping treatment. The pattern was consistent: symptoms peaked around dose increases and eased once the body adjusted. That dose-escalation design (moving through 0.25 mg, 0.5 mg, 1.0 mg, 1.7 mg before reaching 2.4 mg maintenance) exists precisely to reduce the impact of these effects.

The NHS medicines page for semaglutide lists fatigue, headache, dizziness and injection-site reactions alongside the GI group as commonly reported effects. Burping and acid reflux also feature, particularly early on. These are uncomfortable, but for most people they are manageable rather than medically urgent. A slower titration (staying at a lower dose for longer) is sometimes the right clinical call, which is a conversation to have with your prescriber rather than something to adjust on your own.

The bad wegovy side effects that need prompt medical attention

Some effects are less common but serious enough to act on quickly. Acute pancreatitis is the most significant: in January 2026, the MHRA issued a Drug Safety Update specifically flagging GLP-1 medicines and the risk of pancreatitis. The symptom to take seriously is severe stomach pain that does not ease up, particularly if it radiates towards the back, with or without vomiting. That combination warrants an urgent call to 111 or a visit to A&E, not a wait-and-see approach.

Gallbladder problems (gallstones and cholecystitis) are also reported with semaglutide, sometimes without obvious symptoms at first. Persistent pain in the upper right abdomen, fever or yellowing of the skin or eyes should prompt medical review. Signs of a severe allergic reaction (swelling of the face or throat, difficulty breathing, a fast heartbeat) need emergency attention immediately.

Significant GI illness (repeated vomiting or diarrhoea lasting more than a day or two) carries a secondary risk: dehydration can impair kidney function, and some medicines interact differently when you're unwell. Our clinical team at nume is available seven days a week; knowing when to reach out, rather than pushing through, is part of what clinical support is for. There's more detail on timing your injection around illness on our page about taking Wegovy when you're sick.

Less-discussed side effects that patients often find surprising

Beyond the headline GI list, people starting Wegovy sometimes notice things they weren't expecting. Fatigue in the first week or two is among the more common of these, the body is adjusting to slower gastric emptying and, often, eating less. Some report mild hair thinning a few months in; this tends to be linked to rapid weight loss rather than semaglutide directly, and typically reverses as weight stabilises.

There are also some less obviously physical effects. Mood changes and, in some reports, thoughts of self-harm have been noted in post-marketing surveillance; the MHRA and prescribers take these reports seriously. If your mood shifts significantly after starting treatment, tell your prescriber. One less commonly mentioned effect is changes to breath, our page on Wegovy and bad breath covers the likely mechanisms and what tends to help.

For a fuller view of the reported side-effect range (including rarer events) the Wegovy Patient Information Leaflet on the eMC is the authoritative source. It lists every effect by frequency category, and it's worth reading before you start or if you're managing a symptom you can't identify.

How to report a side effect and what our prescribers do with that information

Anyone taking Wegovy in the UK can report a suspected side effect (however minor or uncertain) directly to the MHRA via the Yellow Card scheme. You don't need to be certain it's caused by the medicine; a reasonable suspicion is enough, and reports from patients are a meaningful part of how the regulator monitors medicines after they're approved. Wegovy carries a Black Triangle (▼) designation, meaning additional monitoring is ongoing; patient reports genuinely contribute to that picture.

At nume, our prescribers discuss the side-effect profile as part of every consultation, not as a box-ticking exercise but because how you're tolerating treatment affects what the right next step is. There is no automatic dose increase, no algorithm deciding for you. If you're experiencing bad Wegovy side effects and wondering whether to continue, reduce the dose or pause, that conversation belongs with your prescriber. You can explore what that process looks like on our Wegovy side effects overview, or read about the broader evidence base on the Wegovy treatment page. If you're at the stage of thinking about starting, our free consultation is where that conversation begins, and it covers bad side effects of semaglutide, suitability and your specific circumstances included.