What to Know Before Starting Semaglutide for Weight Loss

Is semaglutide the right medicine for your situation?

Not everyone who wants to lose weight will be suitable for semaglutide, and a prescriber works through that carefully. The licensed eligibility is straightforward on paper: adults with a BMI of 30 or above, or adults with a BMI of 27 to 29.9 who also carry at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. But the number on the BMI calculator is only the starting point.

Your prescriber will also ask about your medical history, past or current pancreatitis, a personal or family history of medullary thyroid cancer or multiple endocrine neoplasia, and any medicines you take regularly. Some conditions mean semaglutide is not suitable; others simply require closer monitoring. Kidney function, gallbladder history, and mental health are all part of the picture. If you are curious about what the full assessment involves, the preparation steps before starting Wegovy are covered in detail elsewhere on the site.

One thing the prescriber will address clearly: semaglutide is not licensed or assessed for cosmetic weight loss, and the MHRA has been explicit about that. The medicine is for people where weight is a clinical health concern, and the prescription reflects that clinical judgement. BMI alone never guarantees approval.

What actually happens to your appetite, and why the slow start matters

Semaglutide is a GLP-1 receptor agonist. It mimics a gut hormone that signals fullness to the brain, slows the rate at which your stomach empties, and reduces appetite at a hormonal level rather than through willpower. That mechanism is why many people find portion sizes that once felt modest now feel genuinely sufficient, the drive to eat more simply quietens.

The titration schedule exists because that same mechanism, introduced too quickly, causes significant nausea. Treatment starts at 0.25mg weekly and steps up through 0.5mg, 1.0mg, 1.7mg, and 2.4mg, each step held for roughly four weeks, or longer if your prescriber advises it. In the STEP 1 clinical trial, participants on the 2.4mg maintenance dose lost an average of around 15% of their body weight over 68 weeks, as published in the New England Journal of Medicine. Those results came alongside a reduced-calorie diet and increased activity, semaglutide works alongside lifestyle changes, not instead of them.

A higher 7.2mg dose was approved by the MHRA in 2026, with trial data showing around 20.7% average weight loss; the titration path to that dose still begins at 0.25mg, which corresponds to the 0.2 ml starting volume used in the pen. For background on how the injection itself is used, guidance on using semaglutide covers technique and routine in more depth.

Which side effects are most likely, and when should you seek urgent help?

The side-effect profile of semaglutide is well-characterised after large clinical programmes. Most people experience some gastrointestinal symptoms, nausea tops the list, followed by loose stools, constipation, indigestion, reflux, and burping. Fatigue and mild headache are also reported. These effects are typically at their strongest after the first injection at a new dose step, then ease as the body settles. Drinking plenty of water and eating smaller, lower-fat meals helps considerably during the adjustment weeks.

There are symptoms that need urgent medical attention rather than watchful waiting. Severe stomach pain that radiates toward the back (with or without vomiting) should prompt a same-day call to NHS 111 or your GP, because pancreatitis, though infrequent, is a known serious risk; the MHRA highlighted this in a Drug Safety Update for GLP-1 medicines in January 2026. Symptoms of an allergic reaction (facial swelling, difficulty breathing, rapid heart rate), or signs of severe dehydration after prolonged vomiting or diarrhoea, also warrant prompt care. Any suspected side effect can be reported at the MHRA's Yellow Card service. If you have questions about your own symptoms between doses, the prescribers at nume's aftercare team are available seven days a week.

What arriving on treatment actually looks like

If you go through a regulated private pharmacy in the UK, semaglutide arrives as a pre-filled Wegovy pen containing four weekly doses. You inject once a week, rotating between the abdomen, thigh, and upper arm. The pen stays in the fridge between uses. When ordered through nume before midday on a working day, your DPD parcel arrives the next working day in plain, unbranded packaging, the tracking notification comes to your phone like any other delivery, with nothing on the outside to indicate what's inside.

Before starting, make sure your GP knows you are beginning treatment; this is standard clinical practice and means your wider health record stays complete. If you are on oral contraceptives, your prescriber will advise adding a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because semaglutide can slow gastric emptying enough to affect pill absorption. Those starting from a previous provider should bring evidence of their current dose. For a full picture of the Wegovy programme, how Wegovy is used for weight loss covers the broader treatment journey. The cost of private treatment varies by dose and provider; what Wegovy costs in the UK explains the pricing landscape honestly. If you would like to see Wegovy weight loss results from people who have been through the programme, before and after accounts are gathered separately on the site. When you feel ready to talk through your own situation, checking your eligibility with our prescribers is a free next step.