The Wegovy side effects list: what happens, when, and what matters

Step 1, The effects that show up first (and most often)

The GI side effects dominate early treatment because semaglutide slows the rate at which your stomach empties. Food sits longer, appetite drops, and for many people so does comfort. The NHS medicines page for semaglutide lists nausea, vomiting, diarrhoea, constipation, indigestion, stomach pain, burping and reflux as very common or common effects. Fatigue, headache and dizziness also appear, though less consistently.

Timing matters here. These effects are most noticeable in the first four to eight weeks and after each dose step. For most people they fade, not immediately, but steadily. Eating smaller amounts, avoiding rich or fatty food during the adjustment window, and staying well hydrated all help. If you find nausea is sharp in the first hour after injecting, some people find switching to an evening dose more manageable; that's a conversation for your prescriber, not a self-directed change.

Injection-site reactions (redness, itching, a small lump) are also listed. Rotating between your abdomen, thigh and upper arm reduces how often these occur. If a site stays inflamed or you notice a hard lump that doesn't settle within a few days, flag it. You can read more about the broader side-effect picture across the treatment on our dedicated Wegovy side effects page.

Step 2, Less common effects worth knowing before you start

Below the common-effects threshold sit a handful of reactions that are infrequent but not trivial. Gallbladder problems (gallstones, inflammation) are among them. Rapid weight loss of any cause is a known risk factor for gallstones, so GLP-1 treatment adds to a background risk that already exists. Symptoms to watch for: upper-right abdominal pain, fever, jaundice (yellowing of the skin or eyes). None of these should be waited out.

Heart rate increase has been recorded in trials. Most people won't notice it; for those with pre-existing heart conditions it's something a prescriber factors into the suitability assessment, not something you need to monitor with a pulse oximeter at home. Hair thinning is reported by some patients, thought to relate to rapid calorie reduction rather than the medicine directly; it typically reverses once weight stabilises.

Mood changes and thoughts of self-harm are listed in the safety information as effects to be aware of. The evidence is still being reviewed across GLP-1 medicines, but the MHRA asks prescribers and patients to stay alert. If your mood shifts noticeably after starting treatment, tell your clinical team promptly. Our guide to the five side effects patients ask about most covers this in more detail.

Step 3, Warning signs that need urgent medical attention

Some things on the side effects list aren't a matter of waiting for them to settle. Know these before your first pen.

Acute pancreatitis: in January 2026 the MHRA issued a Drug Safety Update specifically addressing GLP-1 medicines and pancreatitis, confirming it as an infrequent but potentially serious effect. The symptom to act on is severe, persistent pain in the upper abdomen that spreads to the back, with or without vomiting. If that happens, stop injecting and seek emergency care. Do not wait to see whether it eases.

Allergic reaction: swelling of the face, lips, tongue or throat, difficulty breathing, or a widespread rash after injecting are signs of anaphylaxis. Call 999. Dehydration: prolonged vomiting or diarrhoea can tip into clinically significant fluid loss, which in turn can affect kidney function. If you can't keep fluids down for more than 24 hours, get medical help rather than pushing through. You can find a fuller breakdown of late-stage and persistent effects at our page on side effects that last.

We understand that reading through this list before you've even started can feel daunting. Most people on Wegovy, which you can read more about on our Wegovy treatment page, don't encounter the serious entries. But knowing what to look for puts you in a better position than discovering it mid-crisis.

Step 4 (Reporting, reviewing and what happens if side effects don't settle

Every side effect you experience can be reported through the MHRA's Yellow Card scheme) you don't need to wait for a GP appointment to do it, and you don't need to be certain the medicine caused it. Suspected is enough. Patient reports contribute directly to post-market safety monitoring; for a medicine still under additional MHRA surveillance (Wegovy carries the Black Triangle designation), that data is actively reviewed.

If a side effect is persistent or affecting your daily life, the right move is contact with your clinical team rather than simply stopping the injection. Abrupt discontinuation has its own implications; a prescriber can slow titration, pause at a lower dose, or assess whether continuing makes clinical sense. At nume, prescribers are available seven days a week for aftercare questions, that's the same team that reviewed your original consultation. A question about a symptom doesn't need to become an A&E visit if it's caught early. If you're wondering whether it's possible to experience no side effects on Wegovy, our dedicated page explores what that can mean for your treatment, or explore the full treatment pathway on our weight loss treatments overview.

Our prescribers discuss the side effects list as part of every consultation, it's not a box-ticking exercise. If you have questions before you start, or you're already on treatment and something doesn't feel right, our page on side effects from Wegovy covers what to expect and when to seek support as part of what the service includes.