How long does it take for Wegovy side effects to start?

Step 1, the first 24 to 48 hours: what people notice earliest

The most frequent early reports are nausea and a markedly reduced appetite, both of which can appear within a few hours of the first injection. Semaglutide slows the rate at which the stomach empties, and that physical change is often felt before the day is out. Some people also notice mild bloating, burping, or a vague discomfort in their upper abdomen. Fatigue and mild headache are reported less often but do occur in these first two days.

Not everyone notices anything at all in week one. The starting dose of 0.25mg is calibrated specifically to let the body adjust gradually, so a significant proportion of people find the first injection passes without obvious disruption. If that's your experience, it doesn't mean the medicine isn't working; the lower starting dose is designed for tolerance, not immediate effect. If you're curious about how fast Wegovy side effects typically start, the answer depends on several individual factors worth understanding before your first dose. The timing of first symptoms genuinely varies between individuals, and both early and delayed onset are within the normal range.

One practical note: semaglutide is administered once weekly as a subcutaneous injection, and the pen itself arrives tracked and in plain packaging, nothing on the box identifies its contents. That first injection is often less daunting than people anticipate, but it's worth reading the Patient Information Leaflet (available on the electronic Medicines Compendium) before you start, so you're not caught off guard.

Step 2, the first one to three weeks: when symptoms are most noticeable

For people who do experience digestive side effects, days three to ten are usually when they feel most prominent. Nausea can be persistent rather than sharp, a background queasiness that makes ordinary meals unappealing. Constipation, diarrhoea, and indigestion each appear in a meaningful proportion of users during this window, sometimes in combination. Dizziness and reflux are also reported, particularly after eating.

The full side-effect profile of Wegovy is well documented from the STEP 1 clinical trial, which involved nearly 2,000 adults over 68 weeks and found gastrointestinal effects were the most common reason for early discontinuation, though the majority of participants worked through the initial period successfully. The NHS medicines information for semaglutide describes these as effects that often improve after a few days as the body gets used to the medicine. That timeline (a few days to roughly two weeks) is a reasonable working expectation for most people.

Eating smaller portions, avoiding very fatty or spicy food, and staying well hydrated can help manage symptoms during this window. These are common-sense measures; any specific dietary advice should come from your prescriber or a dietitian, not a general webpage.

Step 3, dose increases: the cycle repeats

Wegovy is titrated through a sequence of doses roughly every four weeks, moving upward as your prescriber judges appropriate. Each increase tends to bring a short recurrence of side effects similar to the ones you experienced at the start, nausea, loose stools, or reduced appetite returning for a few days before settling again. This is a recognised pattern rather than a sign something has gone wrong.

The question of how soon side effects start at a new dose follows a very similar curve to the first injection: often within 24 to 48 hours, peaking in days three to seven, then easing. At higher doses (1.7mg and 2.4mg) the intensity can be slightly greater than at lower ones, which is why the gradual titration schedule exists. If your symptoms feel unmanageable at any stage, that's a conversation for your prescriber before moving to the next dose, not something to push through alone.

One symptom worth knowing about specifically at higher doses is burping. Some people find it more pronounced and persistent than they expected, belching related to Wegovy is a recognised effect of the slowed gastric emptying and is generally harmless, though uncomfortable. It typically eases as the body acclimates.

When to get medical help, and what to report

Most Wegovy side effects are transient and manageable. A smaller number require prompt medical attention. Seek help the same day or call 999 if you experience: severe, persistent stomach pain that radiates toward the back (this can be a sign of pancreatitis (the MHRA issued a specific Drug Safety Update for GLP-1 medicines in January 2026 highlighting this risk); symptoms of a serious allergic reaction such as swelling of the face, lips or throat, or difficulty breathing; signs of severe dehydration from prolonged vomiting or diarrhoea (dark urine, extreme thirst, feeling faint); or gallbladder-type pain) sharp pain in the upper right abdomen, sometimes with nausea or fever.

Low mood or unusual thoughts about self-harm should also be reported to a prescriber promptly. These are rare but listed in prescribing information and taken seriously.

If you want to record and report a suspected side effect formally, the MHRA's Yellow Card scheme accepts reports directly from patients and is one of the ways new safety signals are picked up after a medicine reaches the market. You don't need clinical training to use it. Our prescribers at nume are also available for aftercare seven days a week, if something doesn't feel right, reaching out is always the right call.

A free consultation with our prescribers covers eligibility, side-effect expectations and your individual health picture in full, it's the right place to start.