Wegovy side effects in the first week: separating fact from fear

The biggest misconception: that a rough first week means the medicine is wrong for you

The most unhelpful thing written about Wegovy's first week is that feeling sick means you should stop. It usually means the opposite: your gut is responding to semaglutide exactly as expected, and the 0.25mg starting dose exists precisely to make that response manageable. At this dose, the medicine is not yet working on weight loss in a meaningful way, it is settling your system in for the journey ahead.

Clinical trial data from the STEP 1 study, published in the New England Journal of Medicine, recorded nausea in around 44% of participants taking semaglutide 2.4mg over 68 weeks, but the majority of those reports clustered around dose-escalation points, not sustained across the whole period. The starting dose is considerably lower than 2.4mg, which is why many people find their first week easier than they feared.

The NHS medicines page for semaglutide describes the common side effects plainly: nausea is most likely; vomiting, diarrhoea and constipation are also possible; headache and fatigue appear less often. The pattern is consistent and well-understood. That is worth knowing before day one.

What the first week typically feels like, day by day

Most people inject their first 0.25mg dose and notice nothing for several hours. Nausea, if it comes, usually arrives within the first 24 hours, often around mealtimes, because semaglutide slows the rate at which food leaves the stomach. Eating smaller portions and avoiding fatty or very rich food during this period genuinely helps; that is not a platitude but a practical note grounded in how the medicine works.

By days three to five, appetite is noticeably reduced for many people. Some find this dramatic; others barely notice it at this dose. Constipation is common and sometimes more persistent than nausea, if it becomes uncomfortable, our guide to constipation on Wegovy covers what helps and what to discuss with your prescriber. Loose stools or diarrhoea affect a smaller proportion of starters and usually resolve quickly.

Fatigue and mild headache occasionally appear in days two to four, likely linked to lower calorie intake rather than a direct drug effect. Staying well hydrated and keeping something light in your stomach (a few crackers before the morning injection, for instance) can take the edge off. For a closer look at what to expect across the full seven days, our page on Wegovy week 1 side effects covers what people report day by day. For a closer look at the first 24 hours specifically, our page on first-day Wegovy side effects covers what people report hour by hour.

Symptoms that need medical attention, not tomorrow, now

The vast majority of first-week symptoms are uncomfortable rather than dangerous. But a small group of reactions warrant prompt medical care, and it matters to know the difference before you start.

Severe, persistent stomach pain that radiates toward your back (especially if accompanied by vomiting) needs urgent assessment. The MHRA issued a Drug Safety Update in January 2026 specifically highlighting acute pancreatitis as an infrequent but serious known risk with GLP-1 medicines; the symptom pattern above is the warning sign. This is rare, but act on it quickly if it happens.

Signs of an allergic reaction (swelling of the face, throat or tongue; difficulty breathing; rapid heartbeat; rash) also require immediate emergency help. Severe or prolonged vomiting that leaves you unable to keep fluids down risks dehydration and should prompt a call to 111 or your GP the same day. Injection-site reactions (redness, itching or a small lump at the injection site) are common and almost always harmless; rotate your sites (abdomen, thigh or upper arm) to reduce them. If you are ever uncertain whether a symptom is serious, contact 111 or your prescriber rather than waiting it out. Side effects you suspect are linked to your medicine can be reported to the MHRA through the Yellow Card scheme, which helps regulators monitor real-world safety.

How to approach the first week practically

A few habits make the first week considerably more comfortable. Eat before you feel hungry, waiting until you are ravenous and then eating a full plate is a reliable way to trigger nausea. Smaller, blander meals work better for most people in week one. Keep the pen in the fridge door where you'll see it; out of sight often means missed doses, and consistency in timing helps your body settle into a rhythm.

Alcohol amplifies nausea on semaglutide, particularly in the first weeks. Carbonated drinks can worsen bloating. Ginger, in tea or biscuit form, is a reasonable and evidence-adjacent comfort measure for nausea, though it is not a clinical recommendation, just something our prescribers hear patients mention regularly.

If the first week passes smoothly, weeks two and three typically bring more of the same at the same low dose. Week two's side-effect profile is usually calmer than week one for most people. The pattern shifts again at the first dose increase, which is why week four sometimes feels like a reset of that initial adjustment period, albeit briefer. If you would like to understand the full picture before committing to treatment, our complete Wegovy side effects guide maps the entire titration journey. Wegovy is a prescription medicine; our prescribers go through suitability and side effects with you as part of the consultation, start your free consultation to ask them anything before you begin.