GLP Weight Loss Injections: Evidence, Eligibility and What to Expect

What the trial data established, and why it changed clinical practice

The STEP 1 trial, published in the New England Journal of Medicine, followed 1,961 adults with obesity over 68 weeks. Participants taking semaglutide 2.4mg once weekly alongside lifestyle changes lost an average of around 15% of body weight, roughly three times the loss seen in the placebo group. That result was significant enough to support NICE's subsequent recommendation of semaglutide for weight management in the UK.

Tirzepatide, a dual GIP and GLP-1 receptor agonist, produced even larger average reductions. The SURMOUNT-1 trial, which randomised 2,539 adults with obesity over 72 weeks, reported average body-weight reductions of around 20–21% at the 15mg maintenance dose. NICE appraised tirzepatide and recommended it for NHS use in December 2024 (TA1026), describing the evidence base as robust. The detail that often surprises people: these are average figures across thousands of participants, individual results vary, and both medicines work best alongside dietary and activity changes.

A head-to-head comparison came from the SURMOUNT-5 trial (2025), which found tirzepatide produced greater average weight reduction than semaglutide 2.4mg in adults with obesity who did not have type 2 diabetes. That said, the right injection for any individual depends on their health history, tolerability and what a prescriber judges appropriate, the trial result is a population average, not a personal prediction.

How these GLP injections actually produce their effect

A question our prescribers hear most weeks is some version of: "does the injection just suppress my appetite, or is something more going on?" The honest answer is that several mechanisms work together. Semaglutide activates GLP-1 receptors, which slows gastric emptying (food leaves your stomach more slowly, prolonging fullness) and acts on brain regions involved in hunger signalling. Tirzepatide does the same, and also activates GIP receptors, a second gut-hormone pathway that appears to amplify the appetite-reducing effect and may improve how the body handles blood sugar.

Neither medicine burns fat directly. They alter the biology of appetite so that eating less becomes the path of least resistance rather than an act of sustained discipline. For many people, this is genuinely different from anything they've tried before, and if you want to understand how this class of medicine works in more depth, our page on the GLP-1 weight loss injection covers the science behind it. The NHS medicines page for tirzepatide gives a clear patient-level description of the mechanism and what to expect.

Both medicines are injected once a week into the abdomen, thigh or upper arm, rotating the site each time to avoid skin reactions. Treatment begins at a low starter dose (2.5mg for tirzepatide, 0.25mg for semaglutide) which exists to let the body adjust gradually rather than to produce weight loss from the outset. The prescriber titrates the dose upward, typically in four-week steps, based on how you're tolerating each level. Injection technique matters, and our guide to choosing the best injection site covers the practical details.

Who GLP injections for weight loss are licensed for

Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance, typically 2.5 kg/m² lower for South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds.

Neither medicine is licensed for people under 18, and both are not recommended during pregnancy, breastfeeding or when trying to conceive. Certain conditions (including a personal or family history of medullary thyroid carcinoma, and some gastrointestinal disorders) require careful prescriber assessment before treatment can be considered. BMI alone is never the full picture; a prescriber reviews your whole medical history, current medicines and any relevant conditions before approving treatment. You can read more about how clinical assessment works and what a weight loss injection prescription involves in practice.

On the NHS, access is currently phased and subject to strict criteria, for tirzepatide specifically, eligibility began with people carrying a BMI of 40 or above alongside four or more qualifying conditions. Private treatment through a regulated pharmacy widens access for people who meet the licensed criteria but do not yet qualify under NHS thresholds, and if you want a broader overview of how the options compare, our page covering the range of weight loss injections available is a useful place to start. Our treatment options page explains the full private route.

Side effects and what to watch for

The most common side effects of GLP weight loss injections are gastrointestinal: nausea, loose stools, constipation, indigestion and, less often, vomiting. For most people these are most noticeable in the first few weeks of treatment or after a dose increase, and they tend to ease as the body adjusts. Eating smaller meals, avoiding very fatty or rich food, and staying well hydrated all help.

More serious but less common effects include pancreatitis. In January 2026, the MHRA issued a Drug Safety Update highlighting acute pancreatitis as an infrequent but potentially serious risk with GLP-1 medicines: severe, persistent stomach pain (especially pain that radiates toward the back) needs prompt medical attention. The MHRA Yellow Card scheme allows patients to report any suspected side effect directly, and doing so helps regulators monitor these medicines as real-world use grows.

Women using oral contraceptives alongside tirzepatide should use an additional non-oral method of contraception for the first four weeks of treatment and for four weeks after each dose increase, because slowed gastric emptying may reduce pill absorption. This is a practical point worth raising with your prescriber before starting. For a fuller picture of how these injections compare and what to realistically expect, our page on GLP-1 weight loss injections goes into more detail on how this class of medicine works across different treatments.

If you'd like to explore whether a GLP injection for weight loss is clinically appropriate for you, a free consultation with one of our prescribers is the right starting point, every application is read by a named, GPhC-registered Independent Prescriber, not passed through an automated system. Patients who have been specifically considering tirzepatide may also find it helpful to read through our Waygo weight loss injection page before their consultation.