What happens with your first dose of Wegovy

Step 1 — Choosing the right day and setting yourself up

Pick a day of the week you can keep consistently. Wegovy is injected once weekly, so the first dose sets a rhythm for every pen that follows. Many people choose a day when they are not rushed (a Sunday evening or a quiet weekday morning, for example) so they can pay attention to technique and notice how they feel afterwards.

Before you inject, take a moment to check the pen. Hold it up to the light: the liquid should be clear and colourless with no visible particles. That check takes less than thirty seconds and is worth making every time. Store the pen in the fridge at 2–8°C until you are ready; remove it a few minutes beforehand so it reaches a comfortable temperature. The exact room-temperature window for storage between uses is detailed in the Patient Information Leaflet that comes with your pen, the NHS semaglutide medicines page also gives a reliable overview of what to expect.

Choose an injection site, clean the skin with an alcohol swab, and pinch gently if you are injecting into the abdomen or thigh. Press the pen firmly until you hear the click, hold for the count of ten, and then remove. That is the mechanical process. If anything about your pen looks unusual (discoloured liquid, visible damage to the device) do not use it; contact your prescribing pharmacy instead.

Step 2, What the first dose actually does in your body

Semaglutide is a GLP-1 receptor agonist. It mimics a gut hormone that signals fullness to the brain, slows gastric emptying, and helps regulate appetite over time. At 0.25 mg the pharmacological effect is present but modest; the body is being introduced to the medicine rather than experiencing its full action. This is why clinicians describe the first semaglutide dose as a tolerability phase rather than a treatment phase.

Some people wonder whether anything is working at all in these early weeks. Measurable weight loss tends to come later, once the dose has been increased step by step toward the maintenance level. The titration schedule (0.25 mg, then 0.5 mg, then 1.0 mg, then 1.7 mg, and finally 2.4 mg) is not a suggested pace but a licensed clinical structure. Your prescriber sets the timing. You can read more about how the full schedule unfolds on the Wegovy doses guide.

STEP 1, the pivotal phase 3 trial published in the New England Journal of Medicine, found that adults with obesity lost an average of around 15% of their body weight over 68 weeks on 2.4 mg semaglutide alongside lifestyle changes. That result reflects the full titration, not the first pen.

Step 3, Side effects to know about, and when to act

The side effects most likely to appear after a first Wegovy injection are gastrointestinal: nausea, loose stools, constipation, indigestion, or a feeling of fullness that comes on faster than usual. Most of these are mild, arrive in the first day or two after each new dose level, and ease as the body adapts. Eating smaller, lower-fat meals in the first week can help. Staying well hydrated matters too, particularly if you do experience loose stools.

There is a fuller picture of what to watch for in the first-dose Wegovy side effects guide, but two things warrant prompt attention regardless of when they occur: severe, persistent abdominal pain (especially if it radiates toward the back) should be assessed medically without delay, as it can be a sign of pancreatitis, a recognised but infrequent risk with GLP-1 medicines. The MHRA highlighted this in its January 2026 Drug Safety Update. Signs of a serious allergic reaction (including swelling of the face or throat, or difficulty breathing) also require immediate help.

For everything else, your prescriber and the aftercare team are there. If you are already managing Wegovy and want to understand what the first week of treatment typically looks like in practice, the first week on semaglutide page covers that in detail.

What to expect if you move to a higher dose

After four weeks at 0.25 mg, the next step is 0.5 mg, and, in a real sense, that second dose is another "first" for your body at a new level. The same principle applies at each increase: mild GI symptoms may reappear briefly, then settle. This is why the schedule is paced the way it is. Rushing increases is not clinically advisable, and adjusting your pen to skip a step is not something the device is designed for, the guide on adjusting the Wegovy pen explains this more fully.

Many people find the early months of treatment smoother than they expected. Appetite starts to shift noticeably for most people once the dose reaches 1.0 mg or above. What happens in the first fortnight is mostly about laying the groundwork safely. If you are still weighing up whether Wegovy is the right route and curious about how it compares to other options, the Wegovy overview covers the broader picture. For a sense of what private treatment typically costs, the Wegovy prices page sets out the context honestly.

Wegovy is a prescription-only medicine. If you have not yet spoken to a prescriber about starting treatment, speak to our prescribers through a free consultation, a real clinician reviews your answers the same day.