The Real Difference in Weight Loss Injections Available in the UK

What the trial data actually showed, and what it means

Start with the numbers, because they frame every other comparison. In SURMOUNT-1, published in the New England Journal of Medicine, adults taking tirzepatide 15mg lost an average of around 20–21% of body weight over 72 weeks alongside a reduced-calorie diet and increased activity. The 2025 SURMOUNT-5 head-to-head trial then compared tirzepatide directly against semaglutide 2.4mg in adults with obesity and no diabetes; tirzepatide produced greater average weight reduction over 72 weeks. For semaglutide, the STEP 1 trial (also published in the NEJM) reported an average reduction of around 15% at the 2.4mg maintenance dose over 68 weeks.

A newer semaglutide dose, 7.2mg, was approved by the MHRA in January 2026 and a dedicated single-dose pen was cleared in April 2026. Trials reported around 20.7% average weight loss at 7.2mg over 72 weeks, narrowing (though not closing) the gap with tirzepatide's highest dose. These are population averages from controlled trials, not individual predictions. Some people lose more; some lose less. The prescriber's job, not the numbers, is to decide which medicine fits your situation.

What shifts these averages is, at least partly, mechanism. Tirzepatide activates both GIP and GLP-1 receptors simultaneously, making it the only dual-agonist licensed for weight management in the UK. Wegovy works on GLP-1 alone. Both slow the rate at which food leaves the stomach and act on appetite-regulating pathways in the brain, but the added GIP action in tirzepatide appears to produce greater effects on energy intake and fat storage in clinical populations. If you want to understand how these mechanisms sit within the different types of weight loss injections available in the UK, that page explains the receptor science in more depth.

Dose schedules and what they mean in practice

Both are once-weekly subcutaneous injections, but the titration ladders differ. Mounjaro starts at 2.5mg (a starter dose whose job is to let your system adjust, not to produce therapeutic weight loss) and steps up through 5, 7.5, 10, 12.5 and 15mg, typically in four-week intervals, guided by your prescriber. Wegovy's schedule runs 0.25mg, 0.5mg, 1.0mg, 1.7mg, 2.4mg, and now optionally 7.2mg, also in roughly four-week steps.

Practically, this means the time to reach a meaningful maintenance dose differs between the two medicines, and tolerance of early side effects can vary by individual. Both pens are pre-filled and designed for self-injection into the abdomen, thigh, or upper arm. Sites should be rotated each week. Storage is refrigerated for both; there is a limited room-temperature window set out in each medicine's patient information leaflet, check the leaflet rather than relying on any summary, because the exact window matters if, say, you are away for a long weekend or your order arrives just before a bank holiday. The weight loss injections overview on this site covers storage and administration basics in more detail.

On cost, the two medicines sit at different list prices since Eli Lilly raised Mounjaro's UK pricing from September 2025, the highest-dose pen's list price rose to around £330 per four-week pen, with typical private prices ranging roughly £149–£375 a month depending on dose and provider. The page on cheaper weight loss injection options explains what those figures actually include and why headline price comparisons often miss important context.

NICE criteria and who each medicine is recommended for

NICE has appraised both medicines, and the criteria are not identical. Tirzepatide (TA1026, published December 2024) is recommended for adults with a BMI of 35 or above alongside at least one weight-related comorbidity. Semaglutide (TA875) carries the same headline BMI threshold but specifies use within a specialist weight management service for a maximum of two years, a condition that does not apply to tirzepatide. For both medicines, lower BMI thresholds apply for certain ethnic backgrounds under UK guidance, typically 2.5 kg/m² below the standard figures.

The qualifying comorbidities NICE considers include type 2 diabetes, high blood pressure, dyslipidaemia, obstructive sleep apnoea, and cardiovascular disease. Meeting these conditions does not guarantee NHS access; tirzepatide is being rolled out in phases, and the cohort eligible through GP practices expanded in June 2026. Wegovy on the NHS flows through specialist services, where waiting lists are often long. The treatment overview page sets out both NHS and private routes clearly.

Private eligibility through a regulated online pharmacy is broader than NHS criteria but still governed by the medicines' licensed indications: adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition. BMI alone is not the whole picture. A prescriber reviews your complete health history, current medications, and any contraindications before recommending either medicine, or neither. Our clinical team's approach to that assessment is covered on the clinical profile page.

Side effects and the contraindications that may point you one way or the other

The side-effect profiles of both medicines are broadly similar: nausea, vomiting, diarrhoea, constipation, reflux, headache, and fatigue are the most commonly reported, typically most noticeable after starting or stepping up to a higher dose, and often settling within a couple of weeks. That said, the NHS medicines page for tirzepatide and the equivalent page for semaglutide are the most reliable sources for the full side-effect profile of each, and worth reading before you begin.

One practical difference worth knowing concerns oral contraceptives. For tirzepatide specifically, the MHRA advises adding a non-oral contraceptive method during the first four weeks of treatment and for four weeks after each dose increase, because absorption of the pill may be reduced by slowed gastric emptying. Current guidance does not flag the same concern for semaglutide. If you use combined hormonal contraception, this distinction matters and should be part of your prescriber conversation.

Certain conditions mean neither injectable is appropriate, a personal or family history of medullary thyroid carcinoma or MEN2, active pancreatitis, pregnancy, breastfeeding, or trying to conceive. Under-18s are not covered by either licence. Neither medicine is suitable as a cosmetic treatment, and if you want to understand what the difference between weight loss injections actually means for your contraindication picture, that page is worth reading before your consultation.

If you have read this far and want to explore which option might be clinically appropriate for you, check your eligibility with our prescribers, the consultation is free, and a GPhC-registered clinician reads every submission the same day.