How quickly do Wegovy side effects start — and what to expect

The first 72 hours: what's happening in your body right now

You've just done your first Wegovy injection, or you've moved up to a higher dose, and you're wondering whether that unsettled feeling in your stomach is normal or a warning sign. It's a fair question, and the answer matters.

Semaglutide slows the rate at which your stomach empties, which is part of how it reduces appetite. That same mechanism is also responsible for most of the early side effects. Your gut is adjusting to a different pace, and it tends to let you know about it. Nausea is the most commonly reported symptom and, if you're curious about how fast Wegovy side effects start, it can appear within hours of the first injection for some people, though many feel nothing significant until day two or three. Burping, a heavy or full sensation after small amounts of food, and mild indigestion are all part of the same pattern.

The NHS medicines page for semaglutide lists nausea, vomiting, diarrhoea, constipation, stomach pain and headache as the common early reactions. These are not signs that something has gone wrong. For most people they represent the body's initial response to the medicine, not a reason to stop. You can read more about the full side-effect profile on our Wegovy side effects overview.

Dose increases: why side effects can restart weeks in

One thing that catches people off guard is that side effects can resurface at each dose step, even if the first few weeks were relatively comfortable. The Wegovy titration schedule moves through 0.25 mg, 0.5 mg, 1.0 mg, 1.7 mg and up to 2.4 mg (or 7.2 mg for those on the higher-dose pen approved in 2026), with roughly four weeks between each increase. Every step upward is, in effect, a new adjustment period for the gut.

Nausea and loose stools typically peak in the first one to four days after a dose change and ease over the following week or two as your body adapts. Some less common reactions have a different timing profile entirely, which is why knowing which symptoms can wait and which need same-day attention is worth understanding before you start.

Fatigue, dizziness and headaches are also reported around dose changes. Staying well hydrated matters more than people often realise at this point: reduced appetite can mean reduced fluid intake too, and dehydration compounds most of those symptoms. If diarrhoea is your main issue, the practical guidance on managing Wegovy-related diarrhoea covers approaches your prescriber may discuss with you.

When side effects need medical attention, not patience

Most Wegovy side effects are uncomfortable but self-limiting. A few are not.

Severe, persistent abdominal pain that spreads toward the back (especially with or without vomiting) should not be managed at home. In January 2026 the MHRA issued a Drug Safety Update specifically flagging acute pancreatitis as an infrequent but serious risk with GLP-1 medicines, including semaglutide. The guidance is clear: seek urgent medical help if this type of pain develops. It is not typical start-up nausea.

Other symptoms that warrant prompt contact with a doctor or 111 include: signs of a severe allergic reaction such as facial swelling, difficulty breathing or a widespread rash; symptoms of significant dehydration following repeated vomiting or diarrhoea; persistent vomiting that makes it impossible to keep fluids down; and any sudden vision changes. The timing of when side effects of Wegovy begin matters for knowing which category you're in.

If you're unsure whether what you're experiencing is normal, contacting your prescriber is always the right call. At nume, aftercare is included and available seven days a week. You can reach the team via our contact page or find answers to common questions in our FAQ section.

How to reduce side effects before they start

There are practical steps that genuinely reduce the impact of early Wegovy side effects, and they're worth knowing before your first injection rather than after a rough night.

Eating smaller portions and stopping before you feel full is the single most effective adjustment. Fatty, spicy and rich foods amplify nausea during the adjustment period, so keeping meals plain for the first week or two pays off. Taking the injection on a day when you're not facing a demanding schedule gives your system room to settle without added stress — a detail worth thinking through before you book your first pen.

Alcohol lowers the threshold for nausea and can cause blood pressure fluctuations, so many people find it helpful to cut back substantially during titration. Cold foods often sit better than hot ones when nausea is present, which isn't obvious until someone tells you.

The semaglutide side-effect timeline varies enough between individuals that a prescriber's input is genuinely useful, not just procedurally required. As part of the free consultation at nume's treatment page (checked by a GPhC-registered prescriber, not software) side effects, your medical history and what to watch for are all part of the conversation.