Major weight loss injections: the facts you actually need

What actually happens in your body when you take one of these injections?

Tirzepatide and semaglutide both slow how quickly your stomach empties after a meal. The result is a sustained feeling of fullness that most people notice within the first few weeks. Hunger signals from the brain are also dialled down. The difference is in the receptor targets: semaglutide binds to GLP-1 receptors only, mimicking a hormone your gut releases after eating. Tirzepatide binds to both GLP-1 and GIP receptors, which is why it is the only dual-agonist weight-loss medicine currently licensed in the UK.

You can read a plain-language breakdown of how each of these gut hormones functions on the weight loss injection overview on this site. The short version: neither medicine burns fat directly. They reduce how much you want to eat, which over weeks and months produces a sustained calorie deficit. Protein intake and staying active matter more than many people realise once appetite falls sharply, something our prescribers routinely discuss at review.

Treatment starts at a low dose. The first pen of tirzepatide delivers 2.5mg; the first few weeks on semaglutide begin at 0.25mg. These opener doses are not the therapeutic target, they exist to let your body adjust and to reduce the chance of early nausea. The prescriber then titrates upward in steps, typically every four weeks, based on how you're tolerating treatment. Dosing decisions belong to the clinician, not to a protocol you follow alone.

Who do these injections actually suit, and who should not take them?

UK licensing for both medicines covers adults with a BMI of 30 or above, or 27 and above when at least one weight-related condition is present, high blood pressure, high cholesterol, obstructive sleep apnoea, prediabetes and type 2 diabetes are among those that count. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance, and a prescriber will factor that in.

BMI alone does not decide the matter. A prescriber also looks at your medical history, current medicines and any contraindications. These injections are not recommended during pregnancy, while breastfeeding, or when actively trying to conceive. They are not licensed for anyone under 18. People with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 should not take either medicine. A history of pancreatitis requires careful prescriber discussion before starting.

There are also practical interactions worth knowing. Women taking oral contraceptives who start tirzepatide are advised to add a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because the slowing of gastric emptying can reduce pill absorption. NHS guidance on this is detailed on the weight loss injections guidance page. Semaglutide does not currently carry the same evidence-based warning, though discussing contraception with your prescriber is sensible regardless of which medicine you are prescribed.

What does the clinical trial evidence actually say about results?

The SURMOUNT-1 trial, published in the New England Journal of Medicine, randomised 2,539 adults with obesity and followed them for 72 weeks. At the highest dose, participants lost an average of around 20–21% of body weight. The STEP 1 trial for semaglutide reported roughly 15% average reduction over 68 weeks at the 2.4mg maintenance dose. In the SURMOUNT-5 head-to-head trial, published in the NEJM in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg directly compared over 72 weeks.

A higher 7.2mg semaglutide dose was approved by the MHRA in January 2026, and a dedicated single-dose 7.2mg pen was approved on 14 April 2026. Trials for that dose reported around 20.7% average weight loss over 72 weeks, which narrows the gap with tirzepatide's highest dose considerably. These are trial populations under controlled conditions, individual results vary, which is why the prescriber relationship across months of treatment matters as much as the starting medicine.

For a fuller look at how these two medicines compare across different measures, the detailed injection comparison sets them side by side. The honest conclusion from the evidence is that both work well; which suits any individual is a clinical call, not a consumer preference.

What does taking one of these injections look like week to week?

Each injection is given once a week, subcutaneously, under the skin of your abdomen, thigh or upper arm. You rotate the site each week. The pens are pre-filled; there is no drawing-up or measuring. Changing the needle before each injection is standard practice, and keeping alcohol swabs nearby makes site preparation straightforward.

Storage matters. Both medicines should be kept refrigerated at 2–8°C, the fridge door works, though away from the freezer compartment. Once a pen has been used for its first injection, there is a limited window during which it can be kept at room temperature; the Patient Information Leaflet that comes with your pen states the exact duration, and you should follow that rather than any general figure you read online. The NHS tirzepatide medicine page also covers storage clearly for anyone who wants an official reference.

Side effects are mostly gastrointestinal (nausea, loose stools, constipation, indigestion) and most noticeable in the first days after a new or increased dose. They tend to settle. Severe, persistent stomach pain that radiates to the back is different and warrants urgent medical attention given the known, if infrequent, association with pancreatitis flagged in the MHRA's January 2026 Drug Safety Update. You can find a fuller picture of managing side effects on the weight loss injections and managing treatment page, and any suspected reactions should be reported through the Yellow Card scheme.

If you're weighing up whether a major weight loss injection is clinically appropriate for your situation, including newer options such as the majorino weight loss injection, the starting point is a proper clinical assessment. There is no waiting list, no GP referral and no algorithm reviewing your answers at nume, a GPhC-registered prescriber reads your consultation the same day. For context on treatment costs, the cost guide for weight loss injections explains what a legitimate private prescription should and should not include. When you're ready, speak to our prescribers through a free consultation.