How weight loss injections work: the evidence behind the mechanism

What the trial data established about how these injections reduce weight

The starting point is the clinical evidence, because the mechanism only makes sense once you see what it produces. In the SURMOUNT-1 trial, published in the New England Journal of Medicine, adults taking tirzepatide at the highest dose lost around 20–21% of their body weight on average over 72 weeks, alongside a calorie-reduced diet and activity programme. The STEP 1 trial, also in the NEJM, found semaglutide 2.4mg produced roughly 15% average loss over 68 weeks under similar conditions. These aren't small signals. They're the kind of reductions that, historically, only bariatric surgery reliably achieved.

Both medicines belong to a class called GLP-1 receptor agonists. GLP-1 is a hormone your gut releases after you eat; it tells the brain you're full, prompts the pancreas to release insulin, and slows gastric emptying so food moves through your system more gradually. If you'd like a plain-English explanation of how weight loss injections work at a hormonal level, that page walks through the mechanism in detail. Appetite falls. Meals feel satisfying sooner. The urge to snack between them weakens.

Tirzepatide adds a second layer. It also activates GIP receptors, another gut hormone involved in how fat is stored and how sensitive cells are to insulin. That dual action is widely thought to explain why tirzepatide trials show somewhat larger average losses than semaglutide trials, though the SURMOUNT-5 head-to-head comparison (published in 2025) is the cleanest evidence on that question so far.

Why the stomach-emptying effect matters more than most people expect

Slowing gastric emptying sounds like a detail. In practice, it reshapes the experience of eating. Food that stays in your stomach longer keeps stretch receptors active, which sustains the fullness signal to the brain. Blood sugar rises more slowly after meals, reducing the spike-and-crash cycle that drives hunger a couple of hours later.

That said, the same mechanism is also the source of the most common side effects. Nausea, bloating, and occasional vomiting are reported most often in the early weeks of treatment or after a dose increase, precisely because the digestive system is adjusting to food moving through it more slowly. For most people these effects are mild to moderate and settle within days to two weeks. How quickly the weight-loss effect itself becomes noticeable is a separate question, and one worth reading in detail.

A practical check worth doing before any dose increase: keep a simple note on your phone for the first week at a new dose, three lines, once a day, covering how you felt after meals. It takes under a minute and gives you something concrete to share with your prescriber at review, rather than trying to recall a fortnight's worth of symptoms in one go.

Eligibility: who these medicines are licensed for in the UK

The licences set a clear floor. Tirzepatide is approved for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Semaglutide (Wegovy) carries the same BMI thresholds. An overview of licensed weight-loss treatments in the UK covers both options side by side. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance.

BMI alone doesn't determine suitability. A prescriber looks at the full picture (other medicines you take, relevant medical history, contraindications) before approving treatment. On the NHS, access to these medicines follows phased criteria set by NICE and NHS England; the thresholds are strict and waiting lists can be long. Private treatment through a registered online pharmacy is a route for people who don't meet the current NHS criteria or prefer not to wait, subject to the same clinical assessment. If you want to understand what that clinical review involves at nume, our about page explains how the service is structured.

These medicines are not assessed for quick or cosmetic weight loss, and the MHRA is clear on that point. They are for adults where excess weight is a health risk, used alongside real dietary and activity changes.

The honest answer to whether weight loss injections work

For the population studied in trials (adults with obesity or overweight plus a weight-related condition, using the injections correctly alongside lifestyle support) the evidence says yes, clearly. If you're earlier in your research and still want to understand what weight loss injections actually are before weighing up the evidence, that page covers the basics. The NICE appraisal of tirzepatide (TA1026) reviewed that evidence and recommended the medicine for NHS use, which is not a decision NICE takes lightly.

The qualifier matters, though. Trial results are averages across thousands of participants. Individual outcomes vary. People who stop the medicine typically regain a significant proportion of lost weight over the following year, because the hormonal signal that was suppressing appetite is withdrawn. That's not a flaw in the medicine; it reflects that obesity is a chronic condition shaped by biology. Long-term management (whether that means continuing treatment or transitioning to a structured maintenance plan) is a conversation for a prescriber, not a decision to make from a webpage.

If you'd like to understand the cost side of private treatment before deciding, the weight loss injections cost page sets out what honest pricing looks like and what a transparent price should cover. When you're ready to find out whether you're clinically suitable, start your free consultation and a GPhC-registered prescriber will review your answers the same day.