Side effects after your first Wegovy injection: what happens and when

Step 1, the first 48 hours: what you are likely to feel

The 0.25mg starting dose is chosen specifically to reduce early discomfort. Even so, your digestive system is meeting a new medicine, and it responds. Nausea is the most common experience, often arriving within a few hours of the injection and peaking that first evening or the next morning. Some people also notice a dull headache or mild fatigue. Others feel almost nothing at all.

A practical habit worth building: before you inject, check whether you have eaten a large meal in the past couple of hours. Injecting shortly after a heavy, fatty or spicy meal consistently worsens nausea for many people. A light snack beforehand (or waiting until you have not eaten recently) can take the edge off. It takes under a minute to think through, and it is genuinely useful.

The first Wegovy injection also introduces the mechanics: choosing a site (abdomen, thigh or upper arm), rotating away from any previous spot, and disposing of the needle cap safely. If you are unsure about any step, the Patient Information Leaflet that comes with your pen is the definitive guide, or ask our clinical team.

Step 2, days two to seven: how the side effects normally progress

For most people, nausea is at its worst in the first two to three days and then begins to fade, and if you want to know how long after your first Wegovy injection side effects are likely to start, the timing varies but most people notice something within a few hours. Constipation sometimes appears a little later in the week as gastric emptying slows. Diarrhoea is less common but does occur, particularly if nausea leads people to eat irregularly. Fatigue and mild dizziness can linger a few days longer.

Hydration matters more than usual during this period. Sipping water steadily throughout the day (rather than large amounts at once) tends to sit better on a unsettled stomach. Dry crackers, ginger tea and plain rice are time-honoured nausea remedies that remain practical here. Alcohol is best avoided in the early weeks; it irritates the stomach and compounds dizziness.

The NHS medicines page for semaglutide sets out the full list of recognised side effects and explains which are common versus rare, it is worth reading in full before you start, not after the first twinge of nausea when you are already worried.

Step 3, when to get medical help straight away

Most first-injection side effects are unpleasant rather than dangerous. There are exceptions, and these matter.

Severe stomach pain that does not ease (especially if it radiates toward your back) is the symptom to take seriously. Following a Drug Safety Update issued in January 2026, the MHRA highlighted acute pancreatitis as an infrequent but potentially serious risk with GLP-1 medicines; you can read the full update through the MHRA Yellow Card portal. Call 999 or go to A&E if the pain is severe. Do not wait to see if it passes.

Other signs that need prompt attention: repeated vomiting that prevents you keeping fluids down (risk of dehydration), symptoms of an allergic reaction (swelling of the face, throat or tongue, difficulty breathing), and any thoughts of self-harm or a sudden, significant change in mood.

If something about your reaction feels wrong but does not fit these descriptions, contact your prescriber or call 111. You can also report suspected side effects directly to the MHRA at yellowcard.mhra.gov.uk, this reporting genuinely helps regulators monitor medicines in real-world use, and patients can submit reports themselves.

What changes (and what does not) as the weeks go on

Side effects at each dose level follow a similar pattern to the first injection: the body adjusts over roughly one to two weeks, things settle, and then a dose increase can prompt a milder repeat of the initial response. By the time people reach their maintenance dose, many find they have learned their own triggers and manage them with confidence.

The broader picture of what the side effects of Wegovy injections actually are is covered in detail on our dedicated page, including less common reactions and longer-term considerations, and our pages on semaglutide injection side effects and the side effects specific to the Wegovy injection go deeper into individual reactions if you want to read ahead before your next dose. For a wider look at how semaglutide compares with other injection options, our page on weight loss injections including Ozempic covers the full profile across uses.

One thing that does not change: you remain under clinical oversight. At nume, our prescribers review every repeat order before it is dispensed, if a side effect is affecting your quality of life or you have concerns about your response, the consultation is the moment to raise it. You can also reach our aftercare team seven days a week via the contact page. If you are thinking about starting treatment, check your eligibility and our prescribers will discuss your suitability and what to expect as part of the same-day clinical review.