The main side effects of Wegovy — what to expect and when to worry

The biggest myth: that side effects mean something has gone wrong

One of the questions our prescribers hear most weeks is some version of: "I felt sick on week two, should I stop?" The misconception is that nausea or loose stools signal a bad reaction requiring you to discontinue. In the vast majority of cases they signal the opposite: the medicine is doing what it is designed to do. Semaglutide slows how quickly the stomach empties and acts on appetite centres in the brain. That process is physiologically new for your body, and the gut lets you know about it.

The full side-effect profile for semaglutide is well-characterised from large clinical trials. In the STEP 1 study, published in the New England Journal of Medicine, gastrointestinal events were the most common reason for discontinuation, but they affected a minority of participants overall, and most resolved. The NHS medicines page for semaglutide lists nausea, diarrhoea, vomiting, constipation, stomach pain and burping as the typical profile, categorising them as common or very common at the doses used for weight management.

Practical ways people manage early GI effects: eating smaller portions at mealtimes, avoiding rich or fatty food in the first few weeks, staying well hydrated. None of those are substitutes for talking to your prescriber, dose timing and titration pace are clinical decisions, not self-help adjustments. If side effects feel unmanageable, contact your prescribing service rather than stopping abruptly.

Less common effects worth knowing about

Beyond the GI cluster, several other effects appear with lower frequency but matter enough to understand before you start. Headache, fatigue and dizziness are reported by some people, particularly early in treatment. Injection-site reactions (redness, itching or mild swelling at the abdomen, thigh or upper arm) occur in a proportion of users and usually settle; rotating your injection site each week reduces the risk.

Hair thinning is reported by some people taking GLP-1 medicines, generally attributed to rapid weight loss itself rather than the medicine directly, a pattern seen with other causes of rapid weight change. It typically reverses as weight stabilises. The effects that people find hardest often include fatigue and the psychological adjustment to eating significantly less; both are worth raising with your clinical team.

Low mood and changes in mental wellbeing are listed in Wegovy's product information as effects to monitor. Anyone experiencing thoughts of self-harm should seek urgent help. This is not unique to semaglutide, it is a standard monitoring point for medicines affecting appetite and weight. Your prescriber will ask about mental health as part of the assessment.

For people on oral contraceptives, there is no current evidence that semaglutide reduces pill absorption in the way tirzepatide does, NHS England guidance notes this distinction explicitly. Still, discuss contraception with your prescriber, particularly if starting or changing treatment. Details on how semaglutide compares with tirzepatide in this respect sit on our semaglutide side-effects page.

When to get medical help, and the pancreatitis warning

Certain symptoms need same-day or emergency medical attention. Do not wait to see if they pass.

Acute pancreatitis is an uncommon but serious risk with GLP-1 receptor agonists. In January 2026 the MHRA issued a formal Drug Safety Update for this class of medicines specifically flagging it. The symptom pattern: severe, persistent pain in the upper abdomen that may radiate through to the back, sometimes accompanied by vomiting. If that describes what you are experiencing, stop eating and drinking and call 999 or go to A&E. Do not manage it at home.

Other symptoms requiring urgent attention: signs of a serious allergic reaction (swelling of the face, lips or throat, difficulty breathing, rapid heartbeat); symptoms suggesting gallbladder problems (fever with upper-right abdominal pain, particularly after meals); signs of severe dehydration from persistent vomiting or diarrhoea (dizziness on standing, very dark urine, inability to keep fluids down). Kidney function can worsen if dehydration is prolonged.

Sudden changes in vision should prompt urgent review, particularly in people with pre-existing diabetes. Low blood sugar is not a typical risk with Wegovy alone but can occur in combination with certain diabetes medicines, know the signs if relevant to you.

For a broader look at how common and uncommon effects compare across dose levels, the Wegovy side-effects overview page goes into more detail, and if you want a focused summary you can also read about the main side effects of Wegovy that most people are likely to encounter. The MHRA's Yellow Card scheme is open to patients directly, you do not need a doctor to report a suspected side effect.

How the nume consultation addresses side effects before you start

Understanding what to expect is exactly what a proper prescribing consultation is for. At nume, every consultation is read the same day by a GPhC-registered Independent Prescriber (a real clinician, not automated software) who looks at your full health picture: existing conditions, current medicines, any history that changes the risk-benefit calculation.

Conditions that require careful prescriber assessment or may make Wegovy unsuitable include a history of pancreatitis, gallbladder disease, medullary thyroid carcinoma, and certain gastrointestinal disorders. Wegovy is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. It is not licensed for people under 18.

If you are already taking a GLP-1 medicine elsewhere and want to continue with us, our clinical team reviews evidence of your current dose before confirming any prescription, safety checks apply to transfer patients in the same way as new ones. You can find our clinical team's approach explained on the clinical oversight page.

Side effects are not a reason to avoid treatment if the evidence supports it for you. They are a reason to start under clinical supervision, with a prescriber available when questions come up. Check your eligibility and our prescribers will walk through the suitability assessment with you directly.