How weight loss injections work: what actually happens in your body

Picture the moment you inject, here's what follows

You've taken the cap off, chosen a spot on your abdomen or thigh, pressed the pen against your skin and released the dose. The medicine enters the tissue just beneath the skin and is absorbed gradually into the bloodstream over the following hours. That slow absorption is deliberate: it smooths out the effect across the week, so you're not hit with a surge and then nothing.

From the bloodstream, tirzepatide or semaglutide travels to receptor sites scattered through the body. The gut, the brain's hypothalamus, the pancreas, all of these carry the receptors these medicines are designed to bind to. Once bound, the signalling begins. The stomach is told to slow down. The brain receives a message that food is present even if a full meal hasn't arrived. The pancreas adjusts insulin release in response to meals. These aren't dramatic events you'd feel in the moment; they're shifts in biochemistry that accumulate over days and weeks into a meaningfully different experience of hunger and fullness.

For a fuller breakdown of how each approved medicine compares in its mechanism, our detailed page on how weight loss injections work covers the pharmacology in more depth.

The difference between tirzepatide and semaglutide's mechanism

Both medicines belong to the GLP-1 class, but they're not identical. Semaglutide (the active ingredient in Wegovy) targets GLP-1 receptors only. That single pathway is well-studied and produces meaningful weight loss: in the STEP 1 clinical trial, participants lost an average of around 15% of body weight over 68 weeks, as published in the New England Journal of Medicine.

Tirzepatide adds a second pathway. It also activates GIP receptors, a gut hormone involved in energy storage and appetite regulation that works alongside GLP-1 rather than duplicating it. That dual action appears to amplify the appetite-suppressing effect. In the SURMOUNT-1 trial, participants on the highest dose lost around 20–21% of body weight on average over 72 weeks. Tirzepatide is the only dual GIP and GLP-1 receptor agonist licensed for weight management in the UK, as confirmed by the NHS medicines page for tirzepatide.

The practical upshot: both medicines reduce appetite and slow gastric emptying, but through slightly different molecular routes. Which suits any individual patient is a clinical decision, not a marketing one. If you're weighing the options, our guide to which weight loss injection works the best for different people sets out the evidence side by side.

Why the dose starts low and climbs slowly

This is the part that surprises many people. You begin on a dose that, on its own, isn't expected to produce significant weight loss. For tirzepatide that's 2.5mg; for semaglutide it's 0.25mg. The first pen's job is simply to let your body adjust to the medicine, particularly your gut, which is sensitive to the slower gastric emptying these medicines produce.

Starting low reduces the likelihood of nausea and vomiting in the early weeks. Your prescriber then increases the dose at intervals (typically every four weeks) until you reach the dose that's right for you clinically. The titration schedule is set in the medicine's licensed guidance and is not something to adjust independently; how quickly you notice results depends partly on which dose you're at, which is why patience with the titration process matters.

If you're curious about how results build over time at each stage, our page on how long weight loss injections take to work walks through what the trial data actually shows month by month. The short version: meaningful change typically becomes apparent once a therapeutic dose is reached and sustained.

Side effects as part of the mechanism, not a sign something's wrong

Because these medicines slow gastric emptying and act on the gut, gastrointestinal effects (nausea, loose stools, constipation, burping, indigestion) are the most commonly reported side effects. They arise precisely because the medicine is working on the digestive system. That framing doesn't make them easier to tolerate if you're experiencing them, but it does explain why they tend to be worst at the start of treatment or immediately after a dose increase, then settle as the body adapts.

Serious side effects are rarer. The MHRA has highlighted acute pancreatitis as a known, infrequent but potentially serious risk with GLP-1 medicines: severe stomach pain that spreads to the back, with or without vomiting, warrants prompt medical attention rather than waiting it out. Suspected side effects can be reported at the MHRA's Yellow Card scheme.

Injection-site reactions, fatigue and headache are also reported, particularly early in treatment. Rotating injection sites (abdomen, thigh, upper arm) helps reduce localised irritation. If you're thinking about the practical side of using the pen week to week, our overview of weight loss injections covers what the routine actually looks like in practice. One small thing worth having from the start: a sharps container for safe needle disposal.

If you'd like to discuss whether this treatment is appropriate for your situation, you're welcome to check your eligibility with our prescribers. A real clinician reviews every consultation the same day, no automated decisions.