Semaglutide benefits and side effects: what the evidence actually shows

What the STEP 1 trial tells us about semaglutide's benefits

The clearest evidence for semaglutide's benefits in weight management comes from the STEP 1 trial, a 68-week randomised study published in the New England Journal of Medicine. Adults with obesity or overweight plus a weight-related condition lost an average of around 15% of body weight on the 2.4mg weekly dose, compared with roughly 2.4% on placebo. That is not a marginal difference.

The mechanism is straightforward: semaglutide is a GLP-1 receptor agonist, meaning it mimics a gut hormone that signals fullness to the brain and slows how quickly the stomach empties. Appetite falls. Portions shrink naturally, without white-knuckle restraint. Most participants in STEP 1 were also following reduced-calorie diet and activity guidance, which matters, the medicine works alongside lifestyle changes, not instead of them.

Secondary benefits seen in the trial included improvements in blood pressure, blood sugar regulation, and markers of cardiovascular risk. Whether those improvements apply to you depends on your starting health picture, which is exactly what a prescriber's assessment explores. You can read more about the licensed treatment on the Wegovy overview page.

One practical note: benefits accumulate over months. The first pen is a low tolerability dose. Expecting dramatic early results sets people up for disappointment; steady progress over the full titration schedule is the realistic frame.

The side effects most people actually experience

The NHS semaglutide page lists the common side effects clearly, and they are overwhelmingly gastrointestinal: nausea, vomiting, diarrhoea, constipation, indigestion, burping, and reflux. Fatigue and headache also appear in the data. Most of these are most noticeable in the first few days after starting or after a dose increase, and they tend to settle within one to two weeks as the body adjusts.

That timing matters. Some people stop treatment in week two, when nausea is at its peak, and never reach the dose where meaningful weight loss happens. If a side effect is uncomfortable but manageable, it is worth discussing with a prescriber before deciding to stop. Eating smaller, lower-fat meals, avoiding lying down after eating, and staying well hydrated all reduce the burden for most people.

Less common but worth knowing: some people notice hair thinning a few months in. This is generally linked to rapid weight loss rather than the medicine itself, and it tends to be temporary. Injection-site redness or mild bruising is possible too. For a fuller breakdown of what patients report, our page covering side effects from semaglutide goes into more detail on each one, drawing on what people commonly experience during treatment. You can also visit the semaglutide side effects page for an additional overview.

An unusual symptom worth flagging here: some people notice changes in stool colour or consistency. Green diarrhoea on Wegovy is uncommon but documented, usually reflecting how quickly food is moving through the gut. Discuss anything persistent with your prescriber.

When to get medical help without delay

Most side effects are unpleasant rather than dangerous. A few are not. In January 2026, the MHRA issued a Drug Safety Update specifically calling out acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. The symptom to act on is severe stomach pain that does not settle, particularly if it spreads toward the back and is accompanied by vomiting. That combination warrants same-day medical assessment.

Other reasons to seek prompt help: signs of a serious allergic reaction (swelling of the face or throat, difficulty breathing); symptoms of gallbladder problems (upper-right abdominal pain, fever, jaundice); significant dehydration following prolonged vomiting or diarrhoea; and any new or worsening low mood or thoughts of self-harm. These are not common, but they are on the safety checklist for a reason.

The MHRA asks patients to report suspected side effects through the Yellow Card scheme, it takes a few minutes online and contributes directly to post-market safety monitoring. You do not need a clinician to do it on your behalf. If you have questions about anything you experience, our support team is available seven days a week.

Contraindications and situations that need a frank conversation

Semaglutide is not suitable for everyone. It is not recommended during pregnancy, while breastfeeding, or when actively trying to conceive, the wash-out period before conception should be discussed with your prescriber. Under-18s are outside the licensed age range. A personal or family history of medullary thyroid carcinoma or MEN2 syndrome, and certain pancreatic or gastrointestinal conditions, are reasons a prescriber would decline to prescribe or would want specialist input first.

Oral contraceptives are worth raising at consultation. For tirzepatide there is clear MHRA guidance on adding a non-oral method during dose escalation; for semaglutide, NHS guidance does not flag the same absorption interaction, but your prescriber will factor in your contraception method as part of a thorough review. Similarly, if you are taking HRT or have surgery planned, tell your clinical team.

If you want to understand what to weigh up before starting, our page on semaglutide risks and side effects covers the clinical contraindication list in more depth, alongside the wider safety considerations prescribers take into account. At nume, our prescribers review all of this with you as part of the free consultation, the same-day clinical review is the point at which your health history, medicines, and goals are weighed together, and you can find a thorough overview of what that involves on our page covering semaglutide risks and benefits. That conversation, with a real clinician, is where suitability is established. Speak to our prescribers to start that assessment.