Serious Side Effects of Wegovy You Should Know About

What the clinical evidence and regulators say about Wegovy's serious risks

The STEP 1 trial, published in the New England Journal of Medicine, randomised over 1,900 adults and followed them for 68 weeks. Serious adverse events occurred in a minority of participants, with the gastrointestinal system accounting for most discontinuations. The broader picture from the STEP programme, and from post-marketing experience since Wegovy's UK approval, has shaped the safety information now held on the electronic Medicines Compendium, the authoritative reference for Wegovy's Summary of Product Characteristics.

In January 2026, the MHRA went further, publishing a Drug Safety Update drawing specific attention to acute pancreatitis across all GLP-1 medicines. That update confirmed pancreatitis as an infrequent but potentially serious effect, and asked prescribers and patients to be alert to its symptoms. The MHRA's position is clear: these are not medicines to take without careful clinical assessment and ongoing monitoring.

The NHS medicines page for semaglutide sets out the full side-effect profile in plain language, distinguishing common reactions from those that need immediate attention. It is worth bookmarking that page when you start treatment, glancing at it takes under a minute and keeps the warning signs fresh in your mind.

The warning signs that need same-day or emergency medical attention

Acute pancreatitis is the reaction clinicians are most concerned about. The hallmark is severe, persistent abdominal pain (often felt in the upper abdomen, sometimes radiating through to the back) that does not ease with rest or ordinary pain relief. It may or may not come with vomiting. If this describes what you are experiencing, you need urgent medical attention: call 111, attend an urgent care centre, or call 999 if the pain is severe. Do not wait to see whether it settles overnight.

Gallbladder problems, including gallstones and cholecystitis, are listed in Wegovy's prescribing information as uncommon but documented. Pain in the upper right side of the abdomen, particularly after eating, alongside fever or jaundice, warrants assessment the same day. Kidney problems can arise indirectly, prolonged vomiting and diarrhoea lead to dehydration, which places strain on the kidneys. If you cannot keep fluids down for more than 24 hours, contact your prescriber or 111.

Serious allergic reactions are rare but possible. Swelling of the face, lips, tongue or throat, difficulty breathing, or a rapid rash spreading across the body are signs to treat as an emergency. Call 999. For anyone with a history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2, Wegovy is contraindicated, this is one of the reasons clinical screening before prescribing matters so much. You can read more about the full side-effect spectrum on our detailed Wegovy side effects page.

Reactions that are common and generally manageable

Framing serious effects accurately means also being honest about what is common and not dangerous. The great majority of people using Wegovy report gastrointestinal symptoms at some point: nausea is the most frequent, followed by diarrhoea, constipation, vomiting and reflux. These tend to peak in the days after a dose increase and ease as the body adjusts, usually within one to two weeks. They are the reason the titration schedule exists, moving through doses gradually gives the gut time to adapt.

Headache, fatigue and dizziness appear in the common-effects data too. These are rarely severe and tend not to persist. If you want to read through the complete list, including how frequently each effect was recorded in trials, our page covering all documented Wegovy side effects walks through the prescribing data in full. And if something feels more significant than the usual adjustment discomfort, the page on more troubling Wegovy reactions covers that boundary in detail.

Low mood and thoughts of self-harm have been reported with weight-loss medicines. The causal relationship is not firmly established, but the MHRA has asked that patients and prescribers remain alert. If your mood changes noticeably after starting or increasing your dose, that is worth raising with your prescriber promptly, not at your next routine review, but soon.

Reporting side effects and getting support

Any suspected serious side effect can be reported directly to the MHRA through its Yellow Card scheme. Patients can submit reports themselves, you do not need to go through a doctor first. Yellow Card data actively shapes how regulators update prescribing guidance, which is why reports matter even when a side effect seems unlikely to be connected.

Our clinical team at nume takes the side-effect picture seriously before prescribing and at every repeat review. A named prescriber, not an automated system, reads through your answers and medical history before any treatment is issued. If you have questions about whether Wegovy's side effects during treatment are relevant to your own health situation, or you are concerned about something you are experiencing, our support team is available seven days a week. For a broader look at Wegovy itself, including how it works and who it is licensed for, our guide to what to expect as a side effect of Wegovy is a useful companion before you move on to the Wegovy overview page as the natural next step. If you are weighing up whether treatment is right for you, speaking to our prescribers through a free consultation is how that conversation starts.