Semaglutide risks and side effects: what the evidence actually shows

The biggest myth: 'the side effects are so bad most people stop'

This is the misconception worth addressing first, because it puts a lot of people off treatment that could genuinely help them. In the STEP 1 trial — the pivotal phase 3 study published in the New England Journal of Medicine — around 7% of participants discontinued semaglutide 2.4mg due to adverse events, mainly gastrointestinal ones. That means roughly 93% stayed the course for 68 weeks. The GI effects are common, but for most people they are manageable and time-limited.

What tends to drive the 'unbearable' narrative is the dose-escalation period. Nausea, an unsettled stomach, occasional vomiting or loose stools are most likely when the dose steps up, and our guide to semaglutide side effects explains why these reactions happen and how they typically progress as the body adjusts. Once the body adjusts (usually within a fortnight) they fade for the majority of people. A practical habit worth building: for the first few days on any new dose, take note of what you ate and how you felt an hour later. Keeping even a rough mental log helps you spot patterns (rich or fatty foods often make nausea worse) and gives your prescriber useful information at review.

Guidance on managing nausea on semaglutide covers small meals, pacing, and when to contact your clinical team if symptoms aren't settling.

Side effects that need urgent medical attention

Balanced information means being honest about rarer but serious risks. The NHS medicines page for semaglutide lists the urgent warning signs clearly, and they are worth knowing before you start treatment.

Acute pancreatitis is the most clinically significant. Symptoms include severe, persistent stomach pain that spreads to your back, sometimes with vomiting. It does not feel like ordinary nausea. If that happens, stop treatment and go to A&E or call 999. The MHRA's January 2026 Drug Safety Update formally reinforced this advice for all GLP-1 medicines, including semaglutide. Pancreatitis is uncommon, but it is the side effect where acting quickly matters most.

Other signs to treat as urgent: symptoms of a serious allergic reaction (swelling of the face, tongue or throat, difficulty breathing, rapid heartbeat); signs of gallbladder problems (sharp upper-right abdominal pain, fever, yellowing skin or eyes); and severe dehydration from persistent vomiting or diarrhoea, which can strain the kidneys. Anyone who develops these symptoms should seek medical help promptly rather than waiting for their next scheduled review.

If you experience anything unexpected and want to discuss it, our prescribers are reachable through aftercare support seven days a week.

Risks specific to your situation: who the prescriber looks at more carefully

Semaglutide is not suitable for everyone, and part of why the overall risk picture looks manageable in trials is that trial participants are screened beforehand. The same logic applies in clinical practice.

A personal or close family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia type 2 (MEN2) is a contraindication. A history of pancreatitis warrants careful prescriber discussion. Semaglutide is not licensed for use during pregnancy or breastfeeding, and women who are trying to conceive should talk to their prescriber before starting or continuing. Under-18s are not licensed to use it.

There are also interactions worth flagging. Women taking an oral contraceptive pill alongside tirzepatide face a specific concern about reduced pill absorption during early treatment (less clearly established for semaglutide, but worth discussing). The NHS England guidance on weight management injections recommends telling your anaesthetist about GLP-1 treatment before any surgery. Our clinical team works through all of this at consultation, not as a box-ticking exercise, but because these interactions genuinely shape whether semaglutide is the right choice for you.

For a fuller breakdown of how these risks compare across the licensed options, the Wegovy risks and side effects page covers the injection specifically, while risks of taking semaglutide explores longer-term considerations in more depth. If you are ready to have the suitability conversation, speak to our prescribers through a free consultation.