Wegovy Week 1 Side Effects: What Clinical Evidence and Real Experience Tell You

Why week 1 tends to produce the effects it does, what the trial data showed

Wegovy works by activating GLP-1 receptors in the gut and brain. It slows how quickly the stomach empties and reduces appetite signals. Both of those effects are pharmacologically useful for weight management, and both can cause discomfort when your digestive system encounters them for the first time. The STEP 1 trial, which ran for 68 weeks and compared semaglutide 2.4 mg against placebo in adults with obesity or overweight, reported nausea in roughly 44% of participants on semaglutide across the full titration period, with incidence highest during the early low-dose weeks, well before the therapeutic maintenance doses are reached.

The 0.25 mg dose you start on is not a therapeutic dose in any meaningful clinical sense. Its job is tolerability: easing you into the way the medicine works before your prescriber considers moving you up. That design is deliberate, and it is why many people find week 1 more manageable than they feared. If you want a broader picture of how symptoms tend to shift as your body adjusts, the full Wegovy side effects guide covers the longer arc.

Fatigue and headache appear less often than nausea but are real experiences for some people, particularly in the first few days. Dizziness has been reported too. These effects are thought to reflect the same mechanism: the body recalibrating how it handles food intake and blood glucose movement. Keeping well hydrated and eating smaller, lower-fat meals in week 1 is widely recommended by clinical guidance, your prescriber and the Patient Information Leaflet will give you the specifics that apply to you.

The side effects most people notice in the first seven days, described plainly

Nausea tends to arrive within a few hours of the injection and, for most people, is manageable rather than severe. It can be enough to put you off food, which (combined with the appetite-reducing effect of the medicine itself) sometimes means people eat very little in the first week. That is worth being aware of: under-eating in the early days can amplify nausea rather than relieve it. Small, regular, bland meals tend to help more than skipping them.

Loose stools or diarrhoea are common in the first week, often alongside the nausea rather than instead of it. Constipation is the flip side, some people get one, some the other, occasionally both at different points. Burping and indigestion are less dramatic but frequently mentioned. Injection-site reactions (a little redness or itching where the pen was used) are generally minor and short-lived.

Headache and a general sense of tiredness are reported by a meaningful minority of starters. If you ordered before 12pm on a Monday and your pen arrived the next morning, you may be injecting mid-week, for some people, choosing a day when they can take it easy initially makes the first week more comfortable, and our guide to wegovy side effects in the first week goes into more detail on what to expect during those initial days. The week 2 picture is typically gentler than week 1 for most people, as the body's adaptation begins.

When to get medical help, symptoms that go beyond ordinary adjustment

Most week 1 effects are self-limiting and do not require medical attention. Some are not. The MHRA issued a Drug Safety Update in January 2026 specifically addressing GLP-1 medicines and acute pancreatitis: severe, persistent stomach pain (particularly pain that reaches through to the back, with or without vomiting) requires urgent medical assessment. It is uncommon, but it cannot be attributed to ordinary GI adjustment and must not be waited out.

Seek medical help promptly for: severe abdominal pain radiating to the back; signs of dehydration (very little urine, extreme thirst, dizziness when standing) caused by persistent vomiting or diarrhoea; symptoms suggesting an allergic reaction such as swelling of the face, lips or throat, or difficulty breathing; and any changes in mood, low spirits or thoughts of self-harm that start or worsen after beginning treatment. If you are uncertain whether what you are feeling is within the expected range, contact a clinician rather than waiting. The team at nume's support line is available seven days a week for aftercare questions.

The NHS semaglutide page lists the full set of side effects by frequency and includes clear guidance on which warrant an urgent call. Your Patient Information Leaflet covers the same ground in detail. Anything that worries you can also be reported through the Yellow Card scheme, it matters that post-marketing data reflects genuine patient experience.

How our prescribers approach side effects as part of the consultation

At nume, every consultation is reviewed by a GPhC-registered Independent Prescriber, not automated software. Before treatment begins, they assess your full health picture, existing conditions, current medicines, any history that might affect how you tolerate semaglutide. That assessment shapes whether Wegovy is appropriate for you and, if so, how to approach the start of treatment.

Side effects are part of the clinical conversation, not an afterthought. If you start treatment and find the first week harder than expected, the prescriber can discuss what you are experiencing and whether any adjustment makes sense, and it can also be useful to read ahead about what week 3 Wegovy side effects typically look like so you have a sense of the broader trajectory. No decision about changing, pausing or continuing your treatment should be based on forum posts or a guess, it should come from a clinician who knows your case. Our FAQs cover a range of common early questions, and if you find things settling further, our overview of Wegovy week 4 side effects explains what most people experience as the titration progresses. The free consultation is where those individual conversations start. For a broader sense of what Wegovy involves from the outset, the Wegovy treatment overview is a good place to orient yourself before you begin.