Weight Loss Injections — What Actually Matters Before You Start

The biggest misconception: these injections suppress appetite, not just calories

Most people assume weight-loss injections work the way a very strict diet does, by forcing you to eat less through willpower. That is not what is happening. Mounjaro activates two gut-hormone receptors, GIP and GLP-1, which are involved in signalling fullness and regulating blood sugar. Wegovy activates the GLP-1 receptor alone. Both slow gastric emptying, so food stays in your stomach longer and the sensation of hunger arrives later and more gently. The result is that many people find their relationship with food changes in a way that low-calorie diets alone rarely achieve, not because they are fighting cravings, but because the cravings reduce. This distinction matters because it shapes realistic expectations: the injections support weight loss as part of reduced-calorie eating and increased activity; they are not a substitute for lifestyle change, and the overview of weight-loss approaches on this site puts them in that broader context. It also explains why the side-effect profile is almost entirely gastrointestinal rather than stimulant-related: there is no appetite suppressant in the amphetamine sense. If you want to understand more about how a weight loss injection works at a biological level and what to expect in plain language, it is worth reading before starting.

What the trial data actually shows, and how to read the percentages honestly

The headline figures are real but need context. In the SURMOUNT-1 trial (2,539 adults with obesity, 72 weeks), tirzepatide at 15mg produced an average body-weight reduction of around 20–21%. In the STEP 1 trial (68 weeks, semaglutide 2.4mg), the average was approximately 15%. Those figures come from the SURMOUNT-1 paper in the New England Journal of Medicine and the equivalent STEP 1 publication respectively. A few things the percentages do not tell you: they are averages across large populations, individual results vary considerably, and both trials ran alongside structured lifestyle support. People who stopped treatment regained much of the weight, which is why these are typically described as long-term medicines rather than courses. NICE, in its appraisal of tirzepatide, notes that treatment should be reviewed if less than 5% weight loss has occurred after six months at the highest tolerated dose. The guide to weight-loss injections on this site covers the trial evidence in more depth, including the head-to-head SURMOUNT-5 comparison published in 2025. For cost context, a separate page looks at weight-loss injection pricing in the UK, worth reading if that is part of your decision.

Who these injections are licensed for, and what a UK prescriber assesses

UK licensing sets the floor, not the ceiling of the assessment. The licensed eligibility for both medicines covers adults with a BMI of 30 or above, or a BMI from 27 upwards alongside a weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, obstructive sleep apnoea or cardiovascular disease. For some South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds, UK guidance applies thresholds 2.5 kg/m² lower. But BMI alone is not a ticket. A prescriber also reviews your medical history, current medicines, any contraindications and whether the expected benefit outweighs individual risk. The GP route to weight-loss injections page explains how NHS access works and where private routes differ. These medicines are not suitable during pregnancy, while breastfeeding, or for anyone trying to conceive; they are not licensed for under-18s. Tirzepatide specifically requires women using oral contraceptive pills to add a non-oral method for the first four weeks of treatment and for four weeks after each dose increase, because pill absorption may be affected. That is not a minor detail, it is the kind of thing a prescriber flags at consultation rather than leaving to chance.

Practicalities: storage, injection technique and timing your supply

Once-weekly self-injection sounds daunting to most people who have not done it before. In practice the pre-filled pens used for Mounjaro and Wegovy are designed for home use, a short fine needle, a click mechanism, and a rotating list of sites: abdomen, thigh or upper arm. Keeping the pen refrigerated at 2–8°C is essential; the Patient Information Leaflet specifies a limited room-temperature window, so check it rather than guessing. If you are ordering your next supply through a private pharmacy, plan around any timing gaps: a Monday order placed before 12pm can arrive Tuesday, but bank holidays and payday-adjacent weekends can create brief delays worth factoring in. Needle changes matter, reusing a needle increases the risk of injection-site reactions and blocked tips. If you want to read more about how Mounjaro and Wegovy compare practically, that page covers the differences in pen design and schedule, and there is also a broader comparison of 3 weight loss injections available in the UK if you are still weighing up your options. Safe sharps disposal is a legal requirement; a 1-litre sharps container is an easy addition to your first order. The MHRA's Yellow Card scheme at yellowcard.mhra.gov.uk is the right place to report any suspected side effect, a step the NHS encourages for all medicines under additional monitoring, which both Mounjaro and Wegovy currently are.