What to Expect After Your First Wegovy Injection

The pen is in the fridge, now what happens in your body?

Picture this: you've done your first injection, slid the pen back into the fridge door, and you're waiting to see what happens. For most people, not much changes in the first hour or two. Semaglutide, the active ingredient in Wegovy, works by mimicking a gut hormone called GLP-1, which slows the rate at which food leaves your stomach and sends fullness signals to your brain. That process doesn't switch on instantly, it builds up gradually as the medicine reaches a steady concentration in your system over several days.

By the end of the first 24 hours, you may notice your appetite feels slightly blunted or that smaller portions satisfy you more than usual. Some people notice very little at the 0.25 mg dose. That's expected: the starting dose exists to let your body acclimatise, not to deliver the full effect. The full picture of how Wegovy works includes that titration, the dose steps upward over roughly four months, guided by your prescriber, and appetite suppression strengthens at each stage.

The NHS medicines information for semaglutide covers the mechanism and licensed uses in plain terms, and it's worth reading alongside your Patient Information Leaflet for the specific storage and administration detail that applies to your pen.

Side effects in the first week: what the trial data actually showed

Nausea tops the list. In the STEP 1 clinical trial (68 weeks, over 1,900 adults with obesity) nausea was reported by around 44% of participants taking semaglutide 2.4 mg, compared with roughly 16% on placebo. The important context is that the trial used maintenance doses far higher than the 0.25 mg you start on, and most gastrointestinal side effects were mild to moderate and temporary. They were also most common at the beginning of each new dose level, then eased. What early semaglutide side effects feel like in practice is something many people search before they begin, and the honest answer is: variable, but usually manageable.

Beyond nausea, you might notice vomiting, loose stools or constipation, burping, indigestion, or general fatigue in the days after your first injection. Headache is reported by some people too. Eating smaller meals, avoiding fatty or heavily spiced food, and staying well hydrated all help. Eating slowly and stopping before you feel completely full tends to reduce nausea significantly, your stomach is emptying more slowly than it used to, so there's less room for error at mealtimes.

Injection-site reactions (redness, mild swelling, or a small lump at the site) can occur. Rotating the injection site each week (abdomen, thigh, or upper arm) and reading the technique guidance for your first injection reduces the likelihood. The alcohol swabs supplied with your treatment help keep the site clean.

Red flags: when side effects need more than patience

Most of what you'll feel in the first week is uncomfortable rather than dangerous. But some symptoms require prompt medical attention rather than a wait-and-see approach. In January 2026, the MHRA issued a Drug Safety Update for GLP-1 medicines highlighting acute pancreatitis as a known, infrequent but serious risk: severe stomach pain that doesn't ease and spreads toward your back, with or without vomiting, is the warning sign. If that happens, stop eating and drinking and contact a doctor or call 111 straight away, don't assume it's ordinary nausea.

Other reasons to seek help quickly: signs of an allergic reaction (swelling of the face, lips or throat, difficulty breathing), symptoms of dehydration from repeated vomiting or diarrhoea (feeling faint, dark urine, very low urine output), and any rapid worsening rather than the gradual settling most people experience. You can also report any suspected side effect directly to the MHRA using the Yellow Card scheme, that reporting helps monitor medicines at a population level and is worth knowing about. Our aftercare team is available seven days a week if you have questions about how you're getting on, and you can reach us via the contact page.

What the first injection doesn't change, and realistic early expectations

One thing worth saying plainly: one injection won't produce visible weight loss. The STEP 1 trial's headline result (roughly 15% average body-weight reduction over 68 weeks) reflects months of treatment at maintenance doses, alongside a reduced-calorie diet and regular activity, as published in the New England Journal of Medicine. What the first injection of Wegovy does begin is the process: your appetite signals start to shift, your relationship with portion size starts to change, and the titration schedule your prescriber sets puts you on the path toward effective doses.

If you're weighing up the private cost of treatment, our guide to Wegovy pricing in the UK sets out what honest, all-inclusive pricing looks like and why the cheapest listing isn't always the safest one. For a broader view of weight-loss treatment options, including how semaglutide sits alongside other licensed medicines, the treatment overview is a good next step. And if you'd like to understand the clinical team behind our prescribing decisions, our lead prescriber's profile is there to read. When you're ready to check whether Wegovy is clinically appropriate for you, our page on what happens after your first Wegovy injection is a useful read before you start the free consultation.