How do weight loss injections actually help you lose weight?

What do these injections actually do inside your body?

The two injections licensed for weight management in the UK (tirzepatide (Mounjaro) and semaglutide (Wegovy)) work by activating hormone receptors that your gut uses naturally to signal fullness and regulate blood sugar. After a meal, your body releases hormones called GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). These tell the brain you're full, slow gastric emptying, and reduce appetite for the next meal. The injections replicate and extend those signals.

Semaglutide is a GLP-1 receptor agonist, meaning it binds to one of those receptors. Tirzepatide is the only dual agonist licensed in the UK: it activates both the GLP-1 and GIP receptors simultaneously. That dual action is likely why tirzepatide trials produced larger average reductions, and it also explains why tirzepatide is considered the strongest weight loss injection currently available in the UK. In SURMOUNT-1, participants receiving the 15mg dose lost around 20–21% of body weight over 72 weeks, figures published in the New England Journal of Medicine. The STEP 1 trial for semaglutide 2.4mg showed an average reduction of around 15% over 68 weeks.

Neither injection burns fat directly. What they do is reset the conditions that make overeating easy: appetite falls, portions shrink, and calorie intake drops consistently over months. The weight loss follows from that shift, not from any chemical process that targets fat cells.

Why does slowing your stomach emptying matter for weight loss?

Gastric emptying (the speed at which food moves from your stomach into the small intestine) has a surprisingly large effect on hunger. Food that clears quickly leaves you feeling empty and ready to eat again within an hour or two. These injections slow that process, so a meal occupies your stomach longer, and the hormonal fullness signals it triggers persist well past the point you'd normally reach for a snack.

For people who have spent years feeling hungry despite eating reasonably, this is often the most noticeable early effect. Portions that once felt modest start to feel adequate. That isn't willpower, it's the hormone signalling working as designed. The NHS medicines page for tirzepatide describes this mechanism clearly for patients starting treatment.

One practical knock-on: because gastric emptying slows, alcohol and some oral medicines are absorbed differently. If you take an oral contraceptive pill and start tirzepatide, your prescriber may advise adding a barrier method for the first four weeks of treatment and after each dose increase, since pill absorption may be reduced. It's one of the reasons a clinical consultation (not a simple online order) is the right starting point.

Do the injections keep working, or does the effect fade?

In trials, weight loss continues as long as treatment continues and dose titration progresses. The medicines are typically started at a low dose to help your body adjust, then increased gradually over several months. The therapeutic effect builds as the dose reaches maintenance level, at 2.5mg, tirzepatide is settling your system; the meaningful appetite suppression develops as the dose rises under prescriber guidance.

Stopping treatment tends to reverse the effect. Appetite returns to baseline, and weight regain is common. This is not a personal failure, it reflects the biology. Obesity has recognised physiological drivers, and these medicines address some of them. That's why the question of which injections are most likely to help is best answered as part of a longer conversation about your health history, not a quick pick between products.

Concerns about what happens to muscle during treatment are reasonable too. Evidence suggests the majority of weight lost in trials was fat mass, but protein intake and resistance exercise both matter, something our clinical team flags at consultation. If that's on your mind, there's more detail on how these medicines affect muscle mass.

Are there any side effects you should expect from how they work?

Because the mechanism is GI-based, most side effects are gastrointestinal: nausea, loose stools, constipation, indigestion, burping and occasionally vomiting, particularly in the first few weeks after starting or after a dose increase. For most people they settle within a couple of weeks. Eating smaller meals, staying hydrated, and avoiding fatty or very rich food helps considerably.

Two less common but more serious effects are worth knowing. Pancreatitis (severe, persistent stomach pain that may radiate to the back) requires prompt medical attention; the MHRA issued a specific Drug Safety Update on this in January 2026. Gallbladder problems have also been reported. Both are uncommon but are part of why a prescriber reviews your history before treatment starts, and why aftercare matters throughout.

Hair thinning is sometimes reported and is usually linked to rapid calorie reduction rather than the medicine itself, the evidence on hair and weight loss injections is more nuanced than social media suggests. Keep your pen stored in the fridge door between doses (the same shelf as your milk, typically) and bring it to room temperature before injecting, both small habits that reduce injection-site discomfort. For anything beyond routine questions about side effects, our clinical team is available seven days a week. You can also report suspected adverse effects directly to the MHRA via the Yellow Card scheme.

If you're considering treatment or want to understand whether you'd be eligible, the right next step is a conversation with a clinician. You can explore our available weight loss treatments and speak to our prescribers to get a clear picture of what's appropriate for your situation.