Injectable weight loss drugs licensed in the UK: what the evidence says

What the big trials established, and where the numbers come from

The clinical case for both drugs was built in large, placebo-controlled trials whose results shaped the MHRA's licensing decisions and NICE's subsequent recommendations. The STEP 1 trial, published in the New England Journal of Medicine, followed adults with obesity or overweight over 68 weeks and found that semaglutide 2.4mg produced an average body-weight reduction of around 15% compared with placebo, alongside a reduced-calorie diet and increased physical activity. That result formed the backbone of the UK licence for Wegovy.

Tirzepatide's data came from the SURMOUNT programme. SURMOUNT-1 randomised 2,539 adults and showed average weight reductions of roughly 20–21% at the 15mg dose over 72 weeks, figures that led NICE to recommend tirzepatide for eligible adults in its December 2024 appraisal (TA1026). A 2025 head-to-head trial, SURMOUNT-5, directly compared tirzepatide with semaglutide 2.4mg over 72 weeks and found tirzepatide produced greater average weight loss. That makes it the current answer when people ask about the new injectable drug for weight loss with the strongest trial evidence, though which medicine suits a particular person is a clinical question, not a leaderboard exercise.

One misconception worth clearing up gently: people sometimes wonder whether all weight loss drugs are injectable, and the honest answer is no, though the injectable options currently have the strongest trial evidence behind them. The percentage figures quoted from trials are averages across hundreds or thousands of participants. Individual results span a wide range, and how closely someone follows the accompanying lifestyle guidance matters considerably.

How these injectable drugs act on appetite, and what that means in practice

Both medicines belong to the GLP-1 class, meaning they mimic gut hormones released after eating. They slow gastric emptying (food leaves the stomach more gradually), signal fullness to the brain, and reduce appetite. For many people this means eating noticeably less without a conscious effort to restrict.

The practical difference between the two comes down to receptor targets. Semaglutide acts on a single pathway, the GLP-1 receptor. You can read a plain-English explanation of how that works on our overview of weight loss injectables. Tirzepatide adds a second: the GIP receptor, which also plays a role in energy metabolism. Whether that dual action is the reason for the larger average weight loss in trials is still an active area of research; what the trials measured was the outcome, not definitively the mechanism behind the difference.

Both are taken as a once-weekly subcutaneous injection into the abdomen, thigh or upper arm, rotating sites each week. The injection device is a pre-filled pen, what a weight loss injectable pen looks like and how it works is a question our prescribers field most weeks, and the short answer is that the pen design makes self-injection straightforward for most people after the first attempt. The starting dose in both cases is a tolerability dose: its job is letting your body adjust, not delivering the full therapeutic effect.

Who these medicines are licensed for, and when a prescriber might say no

The MHRA licences both drugs for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition such as high blood pressure, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. NICE's appraisal of tirzepatide sets out the specific NHS eligibility criteria in detail.

Licensing thresholds are a starting point, not an automatic approval. A prescriber will look at your medical history, current medications and other factors, there are contraindications and precautions that make one or both medicines unsuitable for some people. Neither is recommended during pregnancy, breastfeeding or when trying to conceive, and neither is licensed for under-18s. History of certain conditions, including medullary thyroid carcinoma or a history of pancreatitis, requires careful discussion before any GLP-1 medicine is considered.

If you are exploring which approved weight loss injection might be appropriate for you, the right starting point is an honest consultation with a prescriber who can review your full picture, not a comparison of trial averages.

Safety, side effects, and what to do if something feels wrong

The side-effect profile of both medicines is dominated by gastrointestinal symptoms: nausea, loose stools or constipation, indigestion, burping and fatigue are the most commonly reported. These tend to be most noticeable after the first dose or after a dose increase, and typically ease within a week or two as the body adjusts. Starting at a low dose specifically to reduce this early discomfort is why the titration schedule exists.

More serious reactions are rarer but worth knowing. Pancreatitis is a recognised risk: the MHRA issued a Drug Safety Update on GLP-1 medicines in January 2026 highlighting acute pancreatitis as an infrequent but potentially serious effect. Severe, persistent abdominal pain that radiates to the back (with or without vomiting) warrants urgent medical attention. Gallbladder problems, significant dehydration from persistent vomiting or diarrhoea, and signs of an allergic reaction are also reasons to seek help promptly.

Patients can report any suspected side effects directly via the MHRA Yellow Card scheme. It is also worth knowing that counterfeit injectable pens have been seized in significant numbers across the UK, buying from a pharmacy you cannot verify on the GPhC register carries real risk. Checking any online provider at the GPhC pharmacy register takes under a minute and is the single most practical safety step a patient can take. For a broader look at the range of options, our guide to injectable drugs for weight loss covers the landscape in more depth, and our weight loss injections page addresses common questions about the treatment experience itself.

If you have read this far and want to know whether one of these medicines is appropriate for you, a consultation with a GPhC-registered prescriber is the next step. There is no obligation, and no prescription is issued unless the prescriber is satisfied it is clinically suitable. Check your eligibility and start your free consultation.