What weight loss injections actually do, step by step

Step 1: the injection reaches your bloodstream and finds its receptors

Within hours of a subcutaneous injection into the abdomen, thigh or upper arm, the medicine enters circulation and binds to receptors on cells in the gut, pancreas and brain. Semaglutide locks onto GLP-1 receptors. Tirzepatide locks onto both GLP-1 and GIP receptors, the only dual-agonist medicine currently licensed for weight management in the UK. Activating these receptors triggers a cascade: the pancreas adjusts insulin output in response to meals, the stomach slows its rate of emptying, and signals travel to the brain's appetite centres. You can read the clinical detail on how weight loss injections work at a receptor level if you want the mechanism laid out fully.

The GIP pathway that tirzepatide adds may also affect fat cells directly and influence how the body handles energy storage, though research here is still developing. If you are new to this area and want to understand what weight loss injections are and how they differ from older treatments, that background is worth reading before going further. What the trials show clearly is that the combination produces larger average reductions in body weight than GLP-1 activation alone, as the SURMOUNT-5 head-to-head trial demonstrated in 2025.

Step 2: appetite and fullness change noticeably

A question our prescribers hear most weeks is: "Will I just stop feeling hungry, or does something else change too?" The honest answer is both. Gastric emptying slows, so food stays in the stomach longer and stretch-receptor signals reach the brain for more of the meal. At the same time, appetite-regulating signals from the gut change the brain's sense of urgency around food. Most people report smaller portion sizes feeling satisfying within a few weeks of starting, and many notice fewer intrusive thoughts about eating between meals.

These changes are dose-dependent. Treatment starts at a low dose, for tirzepatide that is 2.5 mg, a level designed to let your body adjust rather than to drive weight loss immediately. The prescriber titrates upward, typically in monthly steps, as tolerance builds. If you are still weighing up what the weight loss injections available in the UK actually are, that page sets out the licensed options clearly alongside eligibility criteria. Eligibility for these medicines is assessed against both BMI and any weight-related health conditions you have; the prescriber looks at the whole picture, not the number alone.

Step 3: weight change accumulates over weeks and months

Eating less across each day creates a calorie deficit that, sustained over weeks, produces measurable weight reduction. The medicines do not burn fat directly; they make it significantly easier to maintain that deficit consistently. In the SURMOUNT-1 trial, adults taking tirzepatide 15 mg lost an average of around 20–21% of their body weight over 72 weeks alongside lifestyle changes, as published in the New England Journal of Medicine. Semaglutide 2.4 mg produced around 15% average reduction in the STEP 1 trial over 68 weeks.

Neither figure is a guarantee for any individual. Results depend on how consistently the medicine is used, how closely dietary and activity guidance is followed, and individual biology. The safety profile is important context here too, gastrointestinal side effects, most commonly nausea, are common in the early weeks and can affect how well a person tolerates dose increases. Starting slow is deliberate, not a delay.

For a broader look at how the two licensed options compare, the weight loss treatment overview covers both in plain terms.

Step 4: what the medicine cannot do on its own

The licensed indication for both medicines is clear: they are prescribed alongside a reduced-calorie diet and increased physical activity, not instead of them. The injections change the hormonal environment that governs appetite, but dietary quality still matters, protein intake, hydration and fibre all help preserve lean mass and manage side effects during treatment. Strength-based activity is widely recommended for the same reason.

People sometimes ask whether the effects are permanent. They are not, in isolation. Appetite typically returns to previous patterns if treatment is stopped without corresponding lifestyle changes in place, which is why any responsible prescriber discusses long-term planning, not just the initial prescription. The NHS medicines page for tirzepatide covers the practicalities of treatment, including what happens when you stop. If you are weighing up whether this route is right for you, the weight loss injections overview is a useful starting point before a consultation.

When you are ready to speak to a prescriber, start your free consultation, a GPhC-registered prescriber reads your answers the same day and assesses whether treatment is clinically suitable for you.