How the weight loss injection works — and what happens inside your body

Step 1: the injection itself and how the medicine enters circulation

Both Mounjaro and Wegovy are given as once-weekly subcutaneous injections, meaning just under the skin, not into a muscle or vein. Common sites are the abdomen, thigh, or upper arm, rotated each week to avoid tissue irritation. After injection, the medicine is absorbed slowly into the bloodstream over the following hours, which is part of why a once-weekly dose can produce steady, consistent effects rather than sharp peaks and troughs.

One thing worth clearing up: these are not fat-dissolving injections and they do not target fat cells directly. The mechanism is hormonal, not mechanical. That distinction matters when you're weighing up what to expect.

Once in circulation, the medicine travels to receptor sites in the gut, pancreas, and brain, each playing a different role in the chain of events that follows. If you want to compare the two main options side by side, our overview of weight loss injections available in the UK sets out the key differences plainly.

Step 2: what happens in the gut and brain, the appetite pathway

GLP-1 (glucagon-like peptide-1) is a hormone your body already produces naturally after eating. It signals to your brain that food has arrived, slows the movement of food through your stomach (a process called gastric emptying), and dampens the appetite centres in the hypothalamus. Semaglutide (the active ingredient in Wegovy) works by binding to GLP-1 receptors and replicating that signal far more persistently than your own naturally-produced hormone could.

Tirzepatide, the active ingredient in Mounjaro, does the same and adds a second pathway: it also activates GIP receptors (glucose-dependent insulinotropic polypeptide). GIP is involved in both fat metabolism and appetite regulation, and activating both pathways together appears to produce a stronger effect on body weight than targeting GLP-1 alone. In the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks in adults with obesity.

The result, across both medicines, is that hunger becomes quieter. Portions that previously felt inadequate feel satisfying. The urge to snack between meals reduces. Most people describe it not as feeling sick, but as simply not being very interested in food, a shift that can feel unfamiliar after years of the opposite experience. The evidence on whether weight loss injections actually work explores the trial data behind these effects in more detail.

Step 3: dose titration and why the schedule is gradual

Neither medicine is started at full strength. Mounjaro begins at 2.5mg (a dose designed primarily to let your body adjust before any meaningful therapeutic effect is expected) and is typically increased every four weeks by the prescriber. The same principle applies to Wegovy, which starts at 0.25mg and escalates through several steps to its maintenance dose. This graduated approach exists almost entirely to manage gastrointestinal side effects: nausea, loose stools, and burping are most noticeable when the dose changes, and slowing the schedule down significantly reduces how disruptive they are for most people.

If you are curious about how fast weight loss injections work, appetite often shifts noticeably within the first few weeks, while meaningful weight change usually becomes visible over months, not days. This is a treatment that rewards patience rather than urgency.

For people wondering how the newer weight loss injection, Mounjaro, compares with medicines originally developed for diabetes, the mechanisms overlap significantly, and our page on how diabetes injections work for weight loss goes into that background.

If you are wondering how long weight loss injections take to work, appetite often shifts noticeably within the first few weeks, while meaningful weight change usually becomes visible over months, not days. This is a treatment that rewards patience rather than urgency.

What the treatment does not do, and what you need alongside it

These medicines work best alongside dietary changes and increased physical activity, and that is precisely what the licensed indications specify. They reduce appetite; they do not independently change what your body does with the food you do eat in the way that, say, a fat blocker does. Protein intake, hydration, and some form of regular movement remain important, particularly because rapid weight loss without adequate protein can lead to muscle loss rather than fat loss alone.

The medicines are licensed for adults with a BMI of 30 or above, or from 27 with a weight-related health condition such as high blood pressure, high cholesterol, or type 2 diabetes. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. Suitability, dose, and how long treatment should continue are all clinical decisions, they depend on your full health picture, not just a single number. The fuller clinical picture of how weight loss injections work covers the prescribing framework in more detail.

The NHS medicines page for tirzepatide gives a patient-level overview of how the medicine works, its common side effects, and what to expect during treatment, worth reading before starting. If you're considering your options, our free consultation connects you with a GPhC-registered prescriber who can assess whether this approach is appropriate for you specifically.