Side effects of taking semaglutide: what to expect and when to act

The first few weeks: what most people actually feel

Picture this: your first Wegovy pen has arrived, you've done your first injection on a Sunday evening, and by Tuesday morning you feel mildly queasy before breakfast. That's probably the most typical early experience, and for most people it passes within a few days. The GI effects (nausea, an unsettled stomach, softer stools, occasional reflux) happen because semaglutide slows how quickly food moves through your digestive tract. Your gut is adjusting to a new signal, and it lets you know.

If you want a thorough rundown before you start, our page on what are the side effects of taking semaglutide covers the full list of commonly reported effects, including nausea, vomiting, diarrhoea, constipation, bloating, burping, indigestion and stomach discomfort. Fatigue, headache and mild dizziness also show up in the data. None of those are reasons to stop treatment without speaking to your prescriber first, but they are reasons to eat smaller portions, stay well hydrated and avoid greasy or strongly spiced food while you're settling in. Wegovy-specific side effect guidance goes into the practical management steps in more detail.

One thing that catches people out: the GI effects often return briefly after each dose increase. If you've sailed through 0.5mg, the step to 1mg can feel like starting again, but usually only for a few days. Planning a dose change around a quiet week rather than, say, the Friday before a bank holiday weekend is worth thinking about.

Less common effects that are still worth knowing about

Beyond the GI picture, a smaller proportion of people experience injection-site reactions, redness, mild swelling or itching where the pen was used. Rotating injection sites (abdomen, thigh and upper arm are all suitable) reduces the chance of this becoming a nuisance. Hair thinning sometimes comes up; the evidence on hair loss with Wegovy suggests it's more likely linked to rapid weight loss and reduced calorie intake than to the medicine itself, and it's typically temporary.

Gallbladder problems are less common but appear in the clinical data, gallstones and gallbladder inflammation have been reported with GLP-1 medicines, especially during significant weight loss. If you develop pain in the upper right abdomen, particularly after eating fatty food, tell your GP or prescriber. It's also worth noting that people who become dehydrated through prolonged vomiting or diarrhoea may be at increased risk of acute kidney injury, staying hydrated matters more than it sounds. For a fuller picture of risk considerations, the risks of taking semaglutide page covers this ground in more depth.

The semaglutide side effects overview on our site also pulls together the frequency data from the published trial programme, if you'd like the numbers behind the categories.

When to get medical help: the symptoms that need prompt attention

There is a short list of symptoms that shouldn't wait for your next review appointment. The MHRA's January 2026 Drug Safety Update for GLP-1 medicines singled out acute pancreatitis as an infrequent but potentially serious risk. The symptom to watch for is severe, persistent abdominal pain that spreads towards your back, sometimes with vomiting. If that happens, stop eating and drinking and seek urgent medical care, don't try to manage it at home and don't wait to see whether it passes.

Other symptoms that warrant prompt attention: signs of a serious allergic reaction (swelling of the face, lips or throat; difficulty breathing; rapid heartbeat); significant dehydration that you can't resolve by drinking fluids; any sudden changes in vision. Low mood or thoughts of self-harm should also prompt a call to your GP or 111 without delay, this is flagged in the NHS semaglutide medicines page as something to report to your healthcare team.

If you experience something unexpected that you think might be connected to treatment, you can report it directly to the MHRA through the Yellow Card scheme. Patient reports are a genuine part of how post-market safety is monitored, it's not just a formality.

How prescriber review fits into your side-effect experience

Side effects aren't just a patient-management problem. They're a clinical signal. A prescriber who knows your history can tell the difference between nausea that's expected and transient versus nausea that suggests your dose should stay put rather than increase. That's why Wegovy and all GLP-1 medicines are prescription-only: the titration schedule is personalised, not automatic.

At nume, a GPhC-registered Independent Prescriber reviews every consultation (your own answers, your health background, any medicines you already take) before treatment is approved. Side effects are discussed as part of that conversation, not as an afterthought. If something comes up between orders, our aftercare team is available seven days a week. You can read more about how we work on our about page, or explore the weight-loss treatment overview if you're still weighing your options.

If you're ready to talk through whether semaglutide is right for you, speak to our prescribers through a free consultation, no obligation, just an honest clinical conversation.