Semaglutide Week 1: What the Evidence Says About Starting Treatment

What the STEP 1 trial tells us about the early weeks

The STEP 1 trial is the foundational evidence base for Wegovy. In that 68-week study, participants starting on semaglutide 2.4 mg achieved an average body-weight reduction of around 15% by the end of the trial. That figure is not a week-one number. The trial's design followed the same titration schedule that licensed use still follows today: 0.25 mg for the first four weeks, stepping up through 0.5 mg, 1.0 mg and 1.7 mg before reaching the 2.4 mg maintenance dose.

Why mention the full trial when you're asking about week one? Because understanding the trajectory matters. The first injection is not the one doing the heavy lifting. It is the one that begins the adjustment period, and that context shapes every realistic expectation from this point forward. People who expect a dramatic change in days sometimes lose confidence in a treatment that is, in fact, working exactly as the evidence says it should.

The NHS semaglutide medicines page notes that the dose is titrated gradually to reduce side effects, a detail that also explains why early-stage results are modest. The medicine is being introduced carefully, not held back.

Side effects most people notice in week one

Gastrointestinal symptoms dominate the early experience for many people. Nausea is reported most frequently, followed by loose stools or constipation, burping and a general feeling of fullness that can make meals feel different than usual. Headaches and mild fatigue also appear in the first week for some people, often settling within a few days.

These effects tend to be most pronounced in the 24 to 48 hours after an injection, then ease off. They are not a sign that the medicine is not suiting you, in the majority of cases they are a predictable response to a new mechanism acting on gut-hormone pathways. If symptoms are severe or persistent, the right call is to contact your prescriber rather than adjust the dose yourself.

One practical thing worth knowing: many people find it easier to time their first injection for a day when they do not have a demanding schedule the following morning. If the next day is a school-run day, some people prefer to inject on a Thursday rather than a Sunday so any nausea has passed before the week gets busy. Your prescriber can advise on timing.

For a broader picture of what the following weeks look like, the week two guide covers what changes as the first dose wears off and the second approaches.

Appetite changes in week one: early signals, not steady state

Some people notice a reduction in appetite even at 0.25 mg. Portions feel like enough sooner. The urge to snack between meals is quieter. For others, nothing feels different yet. Both experiences are reported in real-world use, and neither is wrong.

The mechanism (slowing gastric emptying and acting on appetite-regulating centres in the brain) is active from the first dose, but its effect at the starting dose is intentionally gentle. The appetite changes that most people associate with semaglutide treatment become more consistent as the dose titrates upward, and our semaglutide week three guide explores what that next step in the titration typically feels like. If you are wondering how long Wegovy takes to work in a meaningful, sustained way, the honest answer is that weeks rather than days is the right frame.

It is also worth being mindful of protein and hydration during this early period. Reduced appetite is useful, but eating too little too quickly can lead to fatigue and muscle loss alongside fat loss. Your prescriber can point you towards dietary support alongside treatment.

What week one does not tell you, and when to seek advice

A single week on 0.25 mg semaglutide will not predict your eventual results. The one-week results page goes into more detail on this, but the short version is that individual response varies, and the starting dose is designed for safety rather than speed.

There are symptoms that should prompt you to contact a prescriber promptly rather than wait. Severe, persistent stomach pain that radiates towards the back can be a sign of pancreatitis, a known but uncommon side effect that the MHRA highlighted in a January 2026 Drug Safety Update for GLP-1 medicines. Signs of an allergic reaction (swelling, difficulty breathing, significant skin reaction) also require urgent attention. Signs of dehydration following severe vomiting or diarrhoea should not be waited out.

For anything that falls short of those urgent signs but still concerns you, the FAQs cover a range of common questions, and our support team is available seven days a week. Suspected side effects can also be reported through the MHRA's Yellow Card scheme, which helps build the evidence base for medicines in real-world use.

If you are considering starting treatment and want to understand the full cost picture before committing, the Wegovy price page sets out what private treatment typically involves in the UK. When you are ready to speak to a prescriber about whether semaglutide is clinically right for you, the place to start is a free consultation with our team.

Speak to our prescribers about starting semaglutide, consultations are reviewed the same day by a GPhC-registered Independent Prescriber.