Medicine for weight loss by injection: the full picture

You've clicked 'add to basket', then stopped, because something felt off

That hesitation is worth listening to. A medicine for weight loss by injection is not a supplement or a lifestyle product; it's a prescription medicine that changes how your body processes hunger signals. Tirzepatide, sold in the UK as Mounjaro, is a dual GIP and GLP-1 receptor agonist, the only weight-loss medicine in the UK that activates two separate gut-hormone pathways simultaneously. Semaglutide, sold as Wegovy, works on the GLP-1 pathway alone. Both slow gastric emptying and increase the sense of fullness after eating. The clinical trial results are genuine: in the SURMOUNT-1 trial, adults taking tirzepatide at the highest dose lost an average of around 20–21% of body weight over 72 weeks, as published in the New England Journal of Medicine. Semaglutide showed around 15% average loss over 68 weeks in the STEP 1 trial at the then-standard 2.4mg dose.

One misconception that circulates a lot: people assume the injection itself is the complicated part. For most people, using a pre-filled pen once a week turns out to be far less daunting than they expected. The pen is pre-set; you press it against the skin of your abdomen, thigh or upper arm, and the mechanism does the rest. If you want to read through the practical steps before committing to anything, our guide to injecting weight loss medication walks through the process calmly and in detail.

What 'starting at the lowest dose' actually means for you

Both medicines follow a titration schedule, a planned, gradual increase from a low starting dose to a higher maintenance dose. Tirzepatide begins at 2.5mg; the first pen's job is to let your digestive system adapt, not to produce immediate results. The prescriber then reviews and increases the dose, typically every four weeks, working up through 5mg, 7.5mg, 10mg, 12.5mg and 15mg. Semaglutide follows its own stepped schedule up to a maintenance dose. Your prescriber decides the pace based on your response and tolerance, no two people go through it identically.

This matters for anyone comparing prices or looking at what lower-cost injection options actually include. A listing that shows a single dose in isolation rarely tells you whether clinical review, dose-step support and aftercare are part of the package. The NHS England guidance on weight management injections makes clear that safe use of these medicines involves ongoing clinical supervision, not a one-off transaction. At nume, a named GPhC-registered prescriber reads your consultation the same day, and before every repeat, not just at the start.

Who these medicines are licensed for, and what 'eligible' means in practice

The licensed criteria for private prescription broadly follow the medicine's SmPC. Tirzepatide is licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition, such as high blood pressure, high cholesterol, prediabetes or obstructive sleep apnoea. Semaglutide holds the same threshold structure. Lower BMI thresholds can apply for some ethnic backgrounds under UK clinical guidance. If your BMI sits at 27 or 28 and you have a relevant condition, that may still bring you within scope; the prescriber makes the full assessment.

These medicines are not licensed for anyone under 18, and they are not recommended during pregnancy, while breastfeeding, or for those actively trying to conceive. Anyone with a personal or family history of medullary thyroid carcinoma or certain pancreatic conditions will need a careful prescriber conversation before starting. The intersection of weight loss injections and diabetes is worth understanding too, because some of the same medicines are also licensed for type 2 diabetes management, the prescriber's assessment covers your full picture, including any existing conditions and medications. BMI figures are a useful starting point, but they are never the whole story.

Side effects, storage, and the practical realities of weekly injections

The most commonly reported side effects are gastrointestinal: nausea, loose stools, constipation, bloating and, for some people, reflux or burping. These are not universal, and many people find they are most noticeable in the first week or two after starting or after a dose increase, then settle. Fatigue, mild headache and injection-site reactions are also reported. Pancreatitis is a less common but serious risk: if you develop severe, persistent abdominal pain that spreads to your back, seek urgent medical attention. The MHRA issued a Drug Safety Update in January 2026 specifically highlighting this for GLP-1 medicines. You can report any suspected side effects through the Yellow Card scheme.

Storage is straightforward between clinic appointments: both pens are refrigerated at 2–8°C. Each pen contains four weekly doses. For full details on storage windows if the pen has been outside the fridge, always refer to the Patient Information Leaflet that comes with your medicine, a 30-second read that answers these questions precisely. If you'd like to understand how a weight loss medication injection fits into a broader treatment plan, including what to expect at each stage, that page explains the process in full. If you're curious about the wider range of medical weight loss injections available in the UK, including how the licensed options compare, that page covers the landscape. If you want a closer look at how clinically supervised programmes are structured, our page on medi weight loss injections explains what that level of oversight involves and why it matters. And if you're at the point of wanting to understand whether you're a good candidate, our prescribers are ready to assess your situation properly. Start your free consultation and a GPhC-registered prescriber will review it the same day.