What happens in your first week on semaglutide

What the STEP 1 trial tells us about the earliest weeks

The STEP 1 trial enrolled 1,961 adults with obesity or overweight and at least one weight-related condition, running for 68 weeks. Its clearest finding for early treatment is also its most reassuring: the trajectory of weight loss is a slow curve, not a sharp drop. Participants at the 0.25mg dose (the phase everyone begins with) were in an adjustment period, not a therapeutic one. The average body-weight reduction of around 15% accumulated over the full course of treatment, with the steepest losses coming during the higher-dose months.

What this means practically is that week one is not a useful measure of whether semaglutide is working. A person who feels no different by day seven has not failed, and a person who notices reduced appetite by day three has not fast-tracked to success. The body is learning a new hormonal signal, and that takes time. The NHS semaglutide medicines page reflects this framing, noting that most people experience treatment as a gradual process rather than a sudden change.

If you want a fuller picture of how results develop across the first month, the timeline of how long Wegovy takes to work covers each phase in more detail.

Side effects in week one: the pattern, not the panic

Nausea is the side effect most people ask about, and it is genuinely the most common one during early treatment. In STEP 1, gastrointestinal effects were reported most frequently during dose-escalation periods, meaning the 0.25mg starting phase tends to produce milder symptoms than the later jump to 1.7mg or 2.4mg. That said, some people find the first injection brings a dull queasiness that lasts a day or two.

Vomiting, diarrhoea, constipation and indigestion are also on the known list, though most people don't experience all of them simultaneously. Fatigue and mild headache appear occasionally. The standard advice (smaller meals, plain foods, steady fluid intake) is grounded in how the medicine works: semaglutide slows gastric emptying, so a large meal sits longer and can intensify nausea. Keeping a glass of water on the kitchen counter as a visual prompt through the first few days is the kind of small habit that genuinely helps.

Serious symptoms require a different response. Severe, persistent stomach pain that spreads to the back (with or without vomiting) warrants urgent medical attention because, while rare, pancreatitis is a known infrequent side effect of GLP-1 medicines. The MHRA issued a Drug Safety Update in January 2026 specifically covering this risk. Report any side effects you are concerned about via Yellow Card.

Appetite changes: the signal worth paying attention to in week one

Most people who do notice something in the first seven days describe it as appetite rather than weight. Meals feel complete sooner. The mental pull toward snacking is quieter. This is the GLP-1 mechanism beginning to operate, semaglutide mimics a gut hormone that signals fullness to the brain and slows the stomach's emptying rate.

Not everyone feels this early. Some people notice nothing at all at 0.25mg, which is entirely normal and does not predict how they'll respond at higher doses. The dose escalation structure (moving through 0.5mg, 1.0mg, 1.7mg and eventually 2.4mg over several months) exists because the therapeutic effect deepens with each step, and so does tolerability. Week one is the first rung, not the whole ladder.

For a closer look at how things typically progress once the body has settled, the second week of semaglutide and the fourth week cover those phases in their own right. A broader overview of the treatment is on our Wegovy treatment page.

Is semaglutide the right starting point for you?

Wegovy (semaglutide) is licensed in the UK for adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, high cholesterol or prediabetes. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A prescriber assesses the full picture (not just BMI) before treatment is appropriate, and if you're wondering how long it takes to get approved for Wegovy, the process is typically straightforward and can move quickly when you meet the clinical criteria.

The weight-loss treatment options on the nume site cover both Wegovy and Mounjaro, so if you're weighing up which medicine might suit your situation, that's a natural starting point. For questions about accessing semaglutide privately, the guide to getting Wegovy through an online pharmacy explains the clinical process and what legitimate supply looks like in the UK.

A consultation with our prescribers is free, takes place online, and is reviewed by a GPhC-registered Independent Prescriber the same day. If semaglutide is clinically suitable for you, treatment can be dispatched the same day and delivered free the next working day. If you'd like to understand more about our clinical team first, our prescribers page is a good place to start.