How long before side effects of Wegovy appear, and how bad do they get?

The first injection: what the next 48 hours usually feel like

Picture this: you've done your first 0.25mg injection, followed the instructions, and by that evening your stomach feels slightly unsettled. Maybe a little full, maybe mildly queasy. That's the scenario many people describe, and it's reassuring to know it's normal, not a sign something went wrong.

Wegovy works in part by slowing how quickly food moves through your stomach. That's useful for appetite control, but your digestive system takes time to adjust. In the first 24 to 48 hours after each injection, the medicine is reaching its peak concentration in your bloodstream, and the gut tends to register that. Nausea, a reduced appetite, and occasional heartburn are all common at this stage.

A misconception worth setting down gently: many people expect the first injection to cause the most side effects, and then assume it gets worse with every dose increase. In practice, the 0.25mg starter dose is often tolerated well precisely because the dose is low, the prescriber sets it there to let your system adjust. The GI effects often feel more pronounced at the 0.5mg or 1.0mg mark, once the medicine is working harder. That pattern is explained in the full Wegovy side-effect overview on this site.

Fatigue and mild headache in the first couple of days are also reported by some people, probably connected to eating less and the body recalibrating. They usually pass quickly.

Weeks two and three: when symptoms often settle, or spike at a dose increase

By the second week at a given dose, most people find their nausea has softened noticeably. The body is adapting. Appetite suppression often continues (sometimes more strongly) but the acute queasiness tends to recede.

The pattern resets, however, at each dose step. Moving from 0.5mg to 1.0mg, or from 1.7mg to 2.4mg, can feel like a milder version of starting again: a day or two of unsettled stomach, then a gradual easing. Understanding that cycle helps because it means discomfort at week five or nine isn't a sign of treatment failure, it's often just the body catching up to the new level. Our guide on when Wegovy side effects start goes into more detail on the dose-by-dose experience.

Constipation is worth calling out separately. Unlike nausea, it doesn't always peak early and then fade, slower gastric transit can become a persistent feature for some people, especially without adequate hydration and fibre intake. If that's a problem, your prescriber is the right person to discuss it with rather than trying to manage it alone.

Some people also notice hair thinning starting a few months into treatment. This is usually telogen effluvium (a stress response from rapid weight change rather than a direct drug effect) and is covered separately if you want the detail on Wegovy and hair loss.

When to get medical help without waiting

Side effects exist on a spectrum, and most of what Wegovy causes is uncomfortable rather than dangerous. But some symptoms need prompt attention. The MHRA's January 2026 Drug Safety Update specifically flagged acute pancreatitis as a known, if uncommon, risk with GLP-1 medicines: the warning sign is severe stomach pain that doesn't ease and may spread to your back, with or without vomiting. Don't try to wait it out. Go to A&E or call 999.

Other symptoms that warrant same-day contact with a healthcare professional include signs of a serious allergic reaction (swelling of the face, throat or tongue, difficulty breathing), significant dehydration from prolonged vomiting or diarrhoea, and symptoms of gallbladder problems such as sudden sharp pain in the upper right abdomen. A full list of urgent and non-urgent effects to watch for is in your Patient Information Leaflet, and the MHRA Drug Safety Updates page carries the January 2026 communication in full.

If you're unsure whether what you're experiencing needs urgent care or can wait for a routine call, err on the side of caution. That's not overcautious, it's the right instinct with a medicine that directly affects the gut and metabolism. You can also report unexpected or worrying side effects at any point through the Yellow Card scheme.

For questions about how long Wegovy side effects tend to last once they've started, see how long Wegovy side effects last, or if you're wondering when things typically resolve, this page covers the recovery timeline.

Managing side effects: small adjustments, realistic expectations

There's no trick that eliminates GI side effects entirely, but the pattern most people find helpful is straightforward: smaller meals, slower eating, lower-fat and lower-fibre foods in the first days after an injection, and staying well hydrated. Eating a large meal in the hours after your injection is the most reliable way to amplify nausea.

Nausea that arrives when you haven't eaten is a slightly different issue, it's tied to the appetite-suppressing effect creating a genuinely unsettled empty stomach. That's worth reading about if it sounds familiar: Wegovy nausea when hungry covers the mechanism and practical steps.

The cost of treatment and the options available privately are explained on the Wegovy price comparison page if that's on your mind. And if you'd like to understand more about how treatment is structured at nume from the start, the Wegovy overview covers the full picture.

At nume, our prescribers discuss side effects and your individual risk factors as part of every consultation, not as a tick-box, but as a proper clinical conversation. If you'd like to check your eligibility, the consultation is free and reviewed the same day by a real clinician.