Side Effects of Taking Wegovy for Weight Loss

Step 1: What happens in the first few weeks

Wegovy (semaglutide) works partly by slowing how quickly food moves through the stomach. That mechanism is why it reduces appetite, and it's also why the digestive system tends to protest early on. Nausea is the most common complaint, reported by a large proportion of people in the STEP 1 trial published in the New England Journal of Medicine. Constipation and loose stools both appear (sometimes alternating) along with burping, indigestion, and a general sense of fullness that feels uncomfortable rather than satisfying.

Fatigue and mild headaches show up for some people in the opening weeks too, likely connected to eating less and adjusting fluid intake. These are not signals that something has gone badly wrong. They are the body recalibrating.

The NHS medicines page for semaglutide describes these effects in plain language and is worth bookmarking. Injection-site reactions (a small red mark or brief itch) are common and almost always minor. Rotating between the abdomen, thigh, and upper arm, and changing the exact spot each week, reduces the chances of local irritation.

The dose schedule is graduated precisely to keep these effects manageable. Starting at 0.25mg and moving up in stages gives the gut time to adapt before the therapeutic doses arrive.

Step 2: What tends to improve, and what can persist

For many people, nausea fades substantially after the first two to four weeks at any given dose. The pattern repeats each time the dose steps up: a short window of adjustment, then settling. This is a question our prescribers hear most weeks, whether persistent nausea means Wegovy isn't right for them. Usually it means the titration needs more time, not abandonment.

Staying hydrated matters more than it sounds. Nausea suppresses thirst signals, and reduced fluid intake on top of GI losses can accumulate into a dehydration problem. Eating smaller, lower-fat portions and avoiding heavy meals immediately after injecting helps. Timing the injection (some people find evenings work better than mornings) is worth discussing with your prescriber.

Side effects that persist beyond a couple of weeks at a stable dose, or that are severe enough to disrupt daily life, deserve a clinical conversation. The full picture of longer-lasting effects is covered in detail on the long-term side effects of Wegovy page. Gallbladder problems, including gallstones, are a recognised risk with rapid weight loss from any cause and have been reported with semaglutide; right-sided upper abdominal pain warrants prompt medical review.

Hair thinning appears in some people, usually a few months in and related to reduced calorie intake rather than semaglutide directly. It typically resolves. Mood changes and low mood are reported occasionally and should be mentioned to a prescriber if they arise.

Step 3: When to get medical help urgently

Some side effects are uncommon but serious, and speed of response matters. The MHRA issued a Drug Safety Update in January 2026 specifically highlighting acute pancreatitis as an infrequent but potentially severe complication of GLP-1 medicines including semaglutide. The signal to act: severe stomach pain that does not pass, particularly if it radiates toward the back, with or without vomiting. Stop eating, stop your injection, and call 111 or attend A&E. Do not wait for a scheduled call with your prescriber.

Other signs that need same-day or emergency care: symptoms of a serious allergic reaction (swelling of the face, throat or tongue, difficulty breathing, rapid heartbeat), signs of severe dehydration from prolonged vomiting or diarrhoea (dizziness when standing, very dark urine, confusion), and symptoms suggestive of gallbladder inflammation. The risks of taking Wegovy page sets out the full clinical risk picture with greater depth.

If anything feels outside the normal adjustment pattern, the Yellow Card scheme at yellowcard.mhra.gov.uk lets you report a suspected side effect directly to the MHRA. This isn't just a formality. Reporting helps build the safety database for medicines that are still under additional monitoring, Wegovy carries a Black Triangle (▼) precisely because continued post-authorisation evidence is being gathered.

Safety in context: the prescriber's role throughout

Wegovy's side-effect profile is well-documented and, for most people, the effects are front-loaded and manageable. That doesn't mean clinical oversight can stop once a prescription is issued. Wegovy's licensed use for weight management in the UK requires ongoing assessment, and dose increases aren't automatic, they follow a review of how the previous dose was tolerated.

At nume, every repeat order goes back to a GPhC-registered prescriber before it's processed. A clinician reads your update, not a form going into a queue. If you're having a difficult time with side effects between orders, the aftercare team is available seven days a week via our contact page. Questions about whether Wegovy is safe for you (including how side effects interact with your existing medicines or health conditions) are addressed during the free consultation. Our clinical team spends time on these details because the decision to prescribe isn't a checkbox exercise.

If you'd like that conversation, a free consultation with our prescribers is the place to start, they discuss suitability and what side effects to expect from Wegovy as part of the process, before any prescription is written. You can also find a broader overview of semaglutide's side effects in the context of weight loss if you want to read further before booking.