Which weight loss injection is the best one for you?

What do the two UK-licensed weight loss injections actually do?

Mounjaro contains tirzepatide, made by Eli Lilly. It activates two gut-hormone receptors (GIP and GLP-1) that between them signal fullness, slow the rate at which your stomach empties, and influence blood-sugar regulation. It is the only weight-loss injection licensed in the UK that works across both pathways simultaneously. Wegovy contains semaglutide, made by Novo Nordisk, and targets the GLP-1 receptor alone. Both are given as once-weekly subcutaneous injections using a pre-filled pen, and both work best alongside dietary changes and increased activity.

The starting doses are low on purpose. Tirzepatide begins at 2.5mg, a dose whose main job is helping your body adjust, not the dose at which most people see significant weight loss. Semaglutide starts at 0.25mg for the same reason. A prescriber titrates the dose upward roughly every four weeks, based on how well you're tolerating treatment. The full licensed titration schedule, storage guidance and what to do if you miss a dose are all set out in each medicine's Patient Information Leaflet, and your prescriber covers the detail at consultation. The NHS medicines page for tirzepatide gives a clear patient-facing overview of how the medicine works and what to expect.

Which one has better evidence behind it?

This is a question our prescribers hear most weeks, and the short answer is that Mounjaro has produced greater average weight loss in clinical trials, including when compared directly with Wegovy. In the SURMOUNT-1 trial (2,539 adults with obesity, no diabetes, 72 weeks), tirzepatide at the highest dose produced average body-weight reductions of around 20–21%. The STEP 1 trial for semaglutide 2.4mg (68 weeks) reported around 15% on average. Those trials used different populations and designs, so direct comparison between them is imperfect.

A more reliable comparison comes from SURMOUNT-5, a head-to-head trial published in the New England Journal of Medicine in 2025, which randomised over 750 adults with obesity to either tirzepatide or semaglutide 2.4mg for 72 weeks. Tirzepatide produced meaningfully greater average weight reduction, and you can read more about why it is now widely considered the number one weight loss injection among the options currently licensed in the UK. NICE referenced these findings in its appraisal of tirzepatide (NICE technology appraisal TA1026) when it recommended tirzepatide for NHS use in December 2024. It is worth knowing that Wegovy's newer 7.2mg maintenance dose, approved by the MHRA in early 2026, narrows the gap somewhat, with trials reporting around 20.7% average loss at that strength.

Trial averages, though, describe populations. Your own result will depend on how your body responds, your starting point, what you eat, how active you are, and whether you can tolerate the dose increases. A headline percentage is a useful reference, not a personal prediction.

Does 'best' mean the same thing for everyone?

Not remotely. Several factors shift what 'best' looks like in practice. Tolerability is one: some people experience more nausea or gastrointestinal discomfort on one medicine than the other, and a medicine you can stay on consistently will outperform a theoretically stronger one you keep reducing. Other medicines interact differently with each injection, tirzepatide can reduce absorption of oral contraceptives during the first four weeks of treatment and after each dose increase, so women on the pill need to add a barrier method during those periods; there is no equivalent evidence of that effect with semaglutide. If you take oral HRT, the same consideration applies to tirzepatide, and NHS guidance suggests considering transdermal alternatives.

Eligibility also plays a role. Both medicines are licensed for adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance. A prescriber works through your full medical history, current medicines and weight-related conditions before recommending anything. If you are trying to work out which injection might fit your situation before a consultation, our guide on choosing the right injection for you walks through the key considerations in more detail. You can also read about how the two injections compare on current evidence before you speak to a prescriber.

How do you actually get the one that's right for you?

Both Mounjaro and Wegovy are prescription-only medicines. The only lawful route to either in the UK is a prescription issued after a proper clinical assessment, the prescriber, not you, makes the final call on which is appropriate and at what starting dose. That assessment considers your BMI, your medical history, any other medicines you take, and any conditions that might affect suitability or safety.

On the NHS, tirzepatide became available through GP practices from April 2026 under a new prescribing contract, though not all practices have joined and eligibility criteria are strict and phased. Wegovy has been available via specialist weight management services since NICE recommended it in 2023, but waiting lists are commonly long. Private treatment through a regulated online pharmacy is the alternative for people who do not meet NHS criteria yet, or who prefer not to wait. For context on what private treatment costs and what a realistic price includes, our weight loss treatment overview explains how pricing works across the market. You can also explore a full overview of weight loss injections available in the UK and read more on how people rate these medicines in practice. If you want a broader introduction to how a weight loss injection works before deciding whether to pursue a consultation, that page covers the essentials in plain terms.

Whichever route you choose, verify any online pharmacy on the GPhC pharmacy register before handing over personal or medical information. Selling prescription medicines without a valid prescription is illegal, and the MHRA has seized large quantities of fake pens sold through unverified online channels. If a price looks implausibly low or no clinical assessment is mentioned, treat that as a red flag.

If you'd like a prescriber to assess which injection suits your situation, check your eligibility with a free consultation, a real clinician reviews every application the same day.