The strongest weight loss injections in the UK, compared

Which injection produces the greatest weight loss, according to the trials?

The clearest direct answer to this question comes from SURMOUNT-5, a head-to-head trial published in the New England Journal of Medicine in 2025. Over 72 weeks, adults with obesity who took tirzepatide lost more body weight on average than those taking semaglutide 2.4mg. Tirzepatide's edge in that trial was meaningful, not a rounding difference. SURMOUNT-1 had already reported average reductions of around 20–21% at the 15mg dose, across more than 2,500 participants.

Semaglutide has been closing the distance, though. The MHRA approved a 7.2mg Wegovy dose in January 2026 and then a dedicated single-dose 7.2mg pen in April 2026, with trial data showing average weight loss of roughly 20.7% over 72 weeks. That figure puts the highest semaglutide dose much nearer tirzepatide's territory than the original 2.4mg maintenance ever reached. You can read more about both options on our weight loss injections overview.

One practical note worth making: 'strongest' in the trial data refers to averages across large populations. Individual responses vary considerably. Some people lose more than the trial average; others less. The number on a graph does not predict what will happen for any one person, which is exactly why these medicines require clinical oversight rather than a simple self-selection.

What makes tirzepatide different from semaglutide, and does the mechanism explain the results?

Tirzepatide activates two receptors simultaneously: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Semaglutide targets GLP-1 alone. Both pathways are involved in appetite regulation and how quickly food moves through the stomach. The dual action is thought to amplify the appetite-suppressing effect, which many researchers believe partly explains tirzepatide's stronger average results in head-to-head comparisons.

Both are once-weekly subcutaneous injections, self-administered via pre-filled pens, and if you are based locally you can find out how these work in practice on our weight loss injections Harrow page. Tirzepatide's KwikPen contains four doses; Wegovy uses individual pens. Both are Black Triangle (▼) medicines, meaning the MHRA actively collects additional safety data while they are in use. Tirzepatide is made by Eli Lilly; semaglutide by Novo Nordisk.

The Society for Endocrinology notes that GIP receptor activity may also reduce nausea compared with pure GLP-1 stimulation. Whether a patient tolerates tirzepatide more comfortably than semaglutide varies. Gastrointestinal side effects (nausea, loose stools, constipation, indigestion) are the most commonly reported for both. They tend to be most noticeable after a dose increase and often settle within days to a couple of weeks. A prescriber can adjust timing, hydration advice, and dose progression to make the medicine as tolerable as possible.

Does 'strongest' mean it's right for you, and what does eligibility actually look like?

Strength on a chart is one dimension. Clinical suitability is another. 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 health condition such as high blood pressure, raised cholesterol, type 2 diabetes, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone does not guarantee approval, the prescriber looks at the full picture, including existing conditions, other medicines, and any contraindications.

NICE's appraisal of tirzepatide (TA1026) sets out the clinical criteria in detail. There are conditions where neither medicine is appropriate: pregnancy, breastfeeding, and trying to conceive all mean treatment is not recommended. A history of certain thyroid or pancreatic conditions requires careful prescriber discussion. These are not formalities; they are the reason a clinical consultation precedes any prescription.

A quick habit worth building: before ordering from any online pharmacy, spend 30 seconds on the GPhC pharmacy register to confirm it is legally registered. The GPhC registration number should be visible on the pharmacy's site. If it is not, treat that as a red flag. The MHRA has warned repeatedly about counterfeit weight loss pens sold online (some containing entirely different substances) so verifying the pharmacy before you commit is as important as any other step in the process.

Questions about which injection might suit your situation, or about moving between doses, are exactly the kind that our clinical team works through with patients daily. Details on private treatment routes are on our treatments page, where you can start a free consultation. Pricing context, including what a legitimate prescription typically includes, is covered on our weight loss overview.

What should you actually watch for when starting the highest tolerated dose?

Treatment with either medicine begins at a low starter dose, not because it produces meaningful weight loss at that level, but because it allows the body to adjust before moving upward. Doses are increased by the prescriber, typically in four-week steps, based on tolerance, and our weight loss injection page covers what that progression looks like in more detail. Rushing the titration schedule is not beneficial; the upper doses are reached progressively.

The MHRA issued a Drug Safety Update in January 2026 specifically flagging acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. Severe stomach pain that spreads to the back (with or without vomiting) warrants urgent medical attention. Other signs that need prompt review include gallbladder symptoms, significant dehydration following a bout of vomiting or diarrhoea, and any signs of an allergic reaction. Patients and healthcare professionals can report suspected side effects through the Yellow Card scheme.

Injection site rotation (abdomen, thigh, upper arm) reduces local reactions, and you can find a fuller guide to technique and aftercare on our Mounjaro injection for weight loss page. Storage matters too: both medicines need refrigeration, with a limited window for room-temperature storage, and further information on handling is available on our weight loss injection refrigeration page. If you are heading into surgery, tell your anaesthetist about any GLP-1 medicine you are taking.