Which weight loss injection works best at home — and what the evidence actually shows

What the trial data actually measured

When people ask which injection is best, they usually want a number. The trials give us that, with caveats worth understanding. SURMOUNT-1 randomised 2,539 adults with obesity and no diabetes; participants on the highest tirzepatide dose lost around 20–21% of body weight on average over 72 weeks alongside a reduced-calorie diet and increased activity. The STEP 1 trial for semaglutide 2.4mg (also 68 weeks, also alongside lifestyle changes) found an average reduction of around 15%. These are averages across thousands of participants, not individual predictions. Some people lose considerably more, some less, and if you want a fuller breakdown of how the options compare, our guide to what the best weight loss injection is covers the evidence in detail, though the prescriber who reviews your case is better placed than any article to indicate what you might expect.

The head-to-head comparison comes from SURMOUNT-5, an open-label 72-week trial published in the New England Journal of Medicine in 2025, in which tirzepatide produced greater average weight loss than semaglutide 2.4mg. NICE discussed indirect comparisons favouring tirzepatide when it issued its recommendation for tirzepatide in December 2024, NICE technology appraisal TA1026 is the clearest official UK summary of where the evidence sits.

Semaglutide also now has a 7.2mg dose, approved by the MHRA on 14 April 2026 via a dedicated single-dose pen. Early trial data at that dose reported around 20.7% average weight loss, which narrows the gap with tirzepatide substantially. So the picture is still developing, which is partly why NICE and prescribers consider the full clinical picture, not just headline numbers, when recommending one medicine over the other.

How both injections work at home, practically speaking

The practical routine is simpler than many people expect. Both pens come pre-filled; you choose an injection site (abdomen, thigh or upper arm are the standard options), rotate the site each week, and press the pen against the skin. A fresh needle is used every time. If you are new to home weight loss injections, the detailed technique guidance in the Patient Information Leaflet that comes with your pen covers every step, and your prescriber or pharmacist can answer questions before your first dose.

Tirzepatide is supplied as a KwikPen, where each pen contains four weekly doses. Semaglutide's standard pen format is a single-weekly pre-set pen. Both need refrigeration at 2–8°C between uses; the exact room-temperature window for travel or short-term storage is stated in the SmPC for each medicine, always check the leaflet rather than relying on general guidance, because the windows differ. Used needles go into a sharps container; a 1-litre sharps container is the standard size for home use and disposal is via your local pharmacy or council service.

Treatment starts at the lowest dose regardless of which medicine is prescribed. That first stage exists to give your body time to adjust, not because the low dose is expected to produce large results. Titration timing and whether to move up a dose is a decision your prescriber makes (typically reviewed every four weeks) based on how you are tolerating the current level. The NHS tirzepatide patient page covers the titration schedule and what to expect at each stage.

Who is eligible and what the prescription route involves

Both medicines share broadly similar eligibility criteria: adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, dyslipidaemia or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. BMI alone, though, is not the whole picture, a prescriber reviews your full medical history, any medicines you already take, and whether there are contraindications before issuing a prescription.

Neither tirzepatide nor semaglutide is licensed for under-18s. Both are not recommended during pregnancy, while breastfeeding or if you are actively trying to conceive. Women taking oral contraceptives starting tirzepatide should add a non-oral contraceptive method for the first four weeks of treatment and for four weeks after each dose increase, because gastric-emptying changes can reduce pill absorption; this applies specifically to tirzepatide, and NHS guidance notes the evidence for the same effect with semaglutide is less established. If surgery is planned, tell your anaesthetist that you are on one of these medicines in advance.

If you are weighing up your options, our page on which weight loss injection is right for you goes deeper on the individual factors a prescriber considers. For a broader look at all licensed treatments, the overview of weight loss injections available in the UK sets out the full picture.

NHS access, private routes, and a note on cost

Both medicines are available on the NHS, but with real restrictions. Tirzepatide is being rolled out in phases under NICE TA1026: as of summer 2026, NHS prescribing requires a BMI of 40 or above alongside four or more qualifying conditions, with broader eligibility opening from around mid-2026 onwards. Semaglutide is available via specialist weight management services under NICE TA875, with notable waiting lists in most areas. Many people do not yet meet the NHS criteria, or do not want to wait.

Private prescription is the other legal route, through a GPhC-registered pharmacy following a clinical consultation. On price-focused decisions, it is worth reading what a genuine private price typically includes (consultation, prescription review, delivery and aftercare support) because listed monthly costs vary and not all providers are transparent about what is and is not bundled. Our page on weight loss injection pricing context covers the market honestly. And if you find a seller offering either of these medicines without first requiring a clinical assessment, treat that as a serious red flag: the MHRA has repeatedly warned about counterfeit pens circulating online, some containing substances that caused hospitalisations.

To understand the steps from here (consultation, prescriber review, prescription and delivery) how to get weight loss injections in the UK walks through the process clearly. Timing matters less than people think: orders placed by 12pm Monday to Friday, once clinically approved, are dispatched the same day (holidays aside). When you feel ready to take the next step, check your eligibility with our prescribers, the consultation is free, and there is no obligation.