What clinical trials tell us about weight loss injections

Which trials produced the weight loss injection data, and what they actually tested

When people search for clinical trials for weight loss injections, they usually want to know one thing: does the science hold up? The short answer is yes, and it comes from two programmes run in parallel by different manufacturers.

The SURMOUNT programme tested tirzepatide, a dual GIP and GLP-1 receptor agonist made by Eli Lilly. SURMOUNT-1, published in the New England Journal of Medicine, randomised 2,539 adults with obesity or overweight and at least one weight-related condition. Participants at the 15mg maintenance dose lost an average of around 20–21% of body weight over 72 weeks. To put that in human terms: someone starting at 100kg might expect, on average, to reach somewhere in the low 80s. That is not a guaranteed outcome, it is a trial average, and individual results vary.

The STEP programme tested semaglutide 2.4mg, a GLP-1 receptor agonist by Novo Nordisk, later licensed in the UK as Wegovy. STEP 1 ran 68 weeks and reported approximately 15% average reduction in body weight against placebo. Both trials required participants to follow a reduced-calorie diet and increase physical activity, the injection alone was not the only variable.

A 2025 head-to-head, SURMOUNT-5, compared tirzepatide directly against semaglutide 2.4mg in 751 adults over 72 weeks. Tirzepatide produced the greater average reduction. NICE referenced this indirect comparison in its appraisal of tirzepatide (TA1026).

What the trial results do and do not tell you about your own treatment

Trial averages are a useful benchmark; they are not a personal forecast. A participant losing 5% sat in the same trial as one losing 25%, the published figure is the mean. Your starting weight, metabolic profile, the dose you reach, how consistently you follow the lifestyle component, and how well your system tolerates the medicine all shift that number.

The trials also used specific dose schedules, starting low and titrating upward over months. SURMOUNT-1 participants at 15mg had been on treatment for the better part of a year before reaching that dose. The headline result belongs to the whole 72-week period, not to month one.

Searches for clinical trials for weight loss injections near me sometimes reflect a different question: can I join a trial rather than pay privately? Active recruitment trials do exist in UK university hospitals and NHS research sites, but places are limited, eligibility criteria are strict, and you may be randomised to placebo. If your interest is treatment rather than research participation, the practical routes are NHS referral (criteria and waiting times vary, see the NHS weight loss injections guide) or a regulated private clinic.

One practical note: if you are planning a private consultation, it helps to know what weight loss injections clinics typically offer, so you can ask the right questions before you book. Whether you order on a Monday morning or just after payday at the end of the month, same-day clinical review is available Monday to Friday.

How the trial evidence maps to NICE guidance and private eligibility

Regulators and NICE do not just accept manufacturer trial data at face value, they scrutinise methodology, comparators and cost-effectiveness. NICE's appraisal of tirzepatide (TA1026), published December 2024, and its earlier semaglutide appraisal (TA875) both drew directly on the SURMOUNT and STEP trial results to set the eligibility thresholds used in NHS commissioning today.

For private prescribing, the licensed eligibility criteria from those same trials inform who a prescriber can treat: broadly, adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as hypertension, dyslipidaemia, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone does not make someone suitable, a prescriber weighs the whole clinical picture, including any conditions that would make these medicines inappropriate.

If you want to understand how private access works alongside the NHS picture, our page on NHS requirements for weight loss injections sets out the current phased criteria clearly. The options for obtaining weight loss injections in the UK covers both routes in more detail. For a grounding in what these medicines are and how they work, the weight loss treatments overview is a good starting point.

Safety data from the trials, and what to watch for

Clinical trials report adverse events as rigorously as efficacy. Across SURMOUNT and STEP, the most common side effects were gastrointestinal: nausea, vomiting, diarrhoea, constipation, indigestion and reflux. These occurred most often after starting treatment or following a dose increase, and the majority of participants found them mild to moderate and time-limited.

More serious adverse events were infrequent. Pancreatitis is a recognised but uncommon risk with GLP-1 medicines; the MHRA issued a Drug Safety Update in January 2026 highlighting it specifically and advising that severe or persistent stomach pain radiating to the back warrants urgent medical assessment. Reporting any suspected side effects via the MHRA Yellow Card scheme remains the formal mechanism for post-market safety monitoring in the UK, this is how real-world evidence continues to build after a trial ends.

These medicines are not recommended during pregnancy, breastfeeding, or when actively trying to conceive, and they are not licensed for under-18s. Anyone with a history of medullary thyroid carcinoma, MEN2 or certain gastrointestinal conditions should discuss those factors with a prescriber before starting.

For a broader view of what private clinical oversight looks like and who reviews your consultation, the clinical team profile gives a clear picture of how prescribing decisions are made at nume. You can also read more about what to expect from weight loss clinic injections, including how the prescribing process works in practice.