Which weight loss injection produces the best results?

You've read the headlines and now you want a straight answer

You've probably seen competing claims: one article says Mounjaro is the strongest, another says Wegovy is catching up, a third mentions something called Zepbound (a US brand name for tirzepatide that does not exist as a UK product, in the UK it is sold as Mounjaro). It gets confusing fast. So here is what the published evidence actually says, without the noise.

The clearest head-to-head data comes from SURMOUNT-5, a 72-week open-label trial published in the New England Journal of Medicine in 2025, which compared tirzepatide directly against semaglutide 2.4mg in 751 adults living with obesity and no diabetes. Tirzepatide produced the greater average weight reduction. That is the most rigorous direct comparison available, and it is the basis for the best results question being answered with tirzepatide as the current frontrunner.

That said, frontrunner is not the same as right for everyone. The trial studied population averages. A person with a history of pancreatitis, someone on certain oral contraceptives, or someone who finds injections genuinely difficult might be better served by a different approach. Results depend on the dose reached, adherence, and lifestyle factors alongside treatment, all things a prescriber weighs individually.

What makes tirzepatide different from semaglutide, in practice

Semaglutide works on one receptor: GLP-1. It mimics a gut hormone that signals fullness and slows gastric emptying, which is why most people eat noticeably less. That mechanism has produced strong results across the STEP trial programme, STEP 1 alone reported around 15% average weight loss over 68 weeks, as documented by NHS guidance on semaglutide.

Tirzepatide adds a second pathway: GIP receptor activation alongside GLP-1. The combined effect appears to produce stronger appetite suppression and greater average fat mass reduction in trials. SURMOUNT-1 reported around 20–21% average body-weight reduction at 15mg over 72 weeks in adults without diabetes. Comparing these medicines side by side makes the mechanism difference easier to follow.

Wegovy's 7.2mg dose, approved as a dedicated single-dose pen by the MHRA on 14 April 2026, has since added a new dimension. Trial data for 7.2mg reported around 20.7% average weight loss over 72 weeks, a meaningful step up from the 2.4mg figure. The gap between the two medicines is real but smaller at the highest doses than earlier comparisons suggested. An overview of both injections sets out the licensed dose ranges and what to expect at each stage.

What the numbers can and cannot tell you

Clinical trial results are honest averages, not personal predictions. Some participants in SURMOUNT-1 lost considerably more than 20%; others lost less. The same spread exists across STEP trials for semaglutide. Individual response depends on starting weight, metabolic factors, dose tolerance, the lifestyle changes made alongside treatment, and how long someone stays on the medicine.

There is one practical habit worth building early: before each new pen, check the injection site you used last time and rotate, abdomen, thigh, upper arm each offer a different spot. Skin that gets repeatedly injected in exactly the same place can develop small areas of thickening (lipohypertrophy) that reduce absorption and, over months, quietly blunt results. It takes under a minute and makes a measurable difference.

For a fuller picture of what typical results look like in practice, realistic result expectations are covered in more detail, including what the evidence says about timeframes. If you are still deciding which medicine suits your situation, a guide to finding the right fit walks through the clinical factors involved.

Cost is also part of the real-world picture. Eli Lilly raised Mounjaro's UK list price from 1 September 2025, with the highest dose rising from around £122 to around £330 per four-week pen; typical private prices across providers have ranged roughly £149–£375 per month depending on dose and service. Treatment pricing and what is included varies between services, and what a headline price excludes (clinical review, prescription, aftercare, delivery) matters as much as the figure itself.

How a prescriber actually decides which injection to recommend

Neither injection is handed over on the basis of a headline trial figure. A prescriber reviews your BMI, existing conditions, current medicines, contraindications, and preferences. Tirzepatide requires an extra contraception step for women using oral contraceptives during the first four weeks and after each dose increase, NHS guidance confirms absorption of the pill may be reduced, so a non-oral method is added during those windows. Semaglutide does not carry an equivalent documented interaction with the pill. That one clinical detail can influence the recommendation in a way no trial average captures.

Both medicines are licensed 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, or obstructive sleep apnoea. Further detail on how efficacy is assessed across different starting points is available if you want to read more before making a decision.

Our clinical team reviews every consultation personally, a prescriber reads your answers, not a piece of software. If you are ready to find out which injection is the right clinical fit for you, start your free consultation and get an answer from a real clinician, usually the same day.