Not all weight loss injections are the same — here's what actually differs

The myth: 'they're all just appetite suppressants in a pen'

Most people searching this question have seen several injectable medicines mentioned in the same breath (Mounjaro, Wegovy, Ozempic, Saxenda) and assumed the differences are cosmetic: brand name, price, packaging. That assumption is worth correcting before anything else.

Each medicine has a distinct active ingredient, a distinct mechanism, and a distinct UK licence. Ozempic, for example, contains semaglutide but is licensed for type 2 diabetes, not weight management. Saxenda (liraglutide) is licensed for weight loss but works differently again and is not dispensed by our pharmacy. The NHS obesity treatment page distinguishes between these medicines precisely because they are not interchangeable. If you are trying to understand what the weight loss injections are and how they differ, grouping them erases information you need to make a sensible decision.

The two injections currently licensed in the UK specifically for weight management are Mounjaro (tirzepatide, made by Eli Lilly) and Wegovy (semaglutide, made by Novo Nordisk). Both are prescription-only, both are once-weekly subcutaneous injections, and both are used alongside a reduced-calorie diet and increased physical activity. That is roughly where the similarities end.

What the mechanism difference actually means in practice

Mounjaro is a dual GIP and GLP-1 receptor agonist, the only one of its kind licensed for weight loss in the UK. Activating both receptors affects appetite signalling, gastric emptying, and blood-sugar regulation through two complementary pathways. Wegovy is a GLP-1 receptor agonist only, working through a single pathway that also slows stomach emptying and reduces appetite.

This is not a theoretical distinction. In the SURMOUNT-5 head-to-head trial (751 adults with obesity, no diabetes, 72 weeks) tirzepatide produced a greater average weight reduction than semaglutide 2.4 mg, as published in the New England Journal of Medicine (2025). SURMOUNT-1 separately reported average body-weight reductions of around 20–21% at the 15 mg tirzepatide dose over 72 weeks. STEP 1, the equivalent semaglutide trial, reported roughly 15% average weight loss at the 2.4 mg maintenance dose over 68 weeks. Those are population averages from clinical trials, not guarantees for individuals, but the gap is large enough that a prescriber weighs it when recommending one over the other.

A higher-dose semaglutide option also exists: the MHRA approved a dedicated 7.2 mg Wegovy pen in April 2026, with trial data showing around 20.7% average weight loss over 72 weeks, which narrows the gap considerably. If you want to understand what all the weight loss injections are and how the full range compares, that is worth reading before settling on a format. Which dose, and which medicine, suits you is a clinical judgement, not something a comparison article can settle.

How the eligibility and NHS access picture splits between them

Both medicines share a broadly similar private licensed threshold: adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, high cholesterol, type 2 diabetes, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone is not sufficient; a prescriber reviews your full medical history.

On the NHS the picture is very different, and different for each medicine. Wegovy is available via specialist weight management services under NICE guidance (TA875), for a maximum of two years, with multidisciplinary support; waiting lists are commonly long. Tirzepatide was recommended by NICE in December 2024 (TA1026) for adults with a BMI of 35 or above plus at least one weight-related comorbidity, with NHS rollout phasing in by cohort from 2025. From April 2026, some GP practices can prescribe tirzepatide under the new GP contract, though participation varies by area. If you want a fuller picture of the cost side of private treatment, our weight-loss overview sets that out plainly.

The practical takeaway: if you do not meet the current NHS criteria or prefer not to wait, a private prescription from a regulated pharmacy is the alternative, subject to the same clinical assessment. You can read NICE's tirzepatide appraisal (TA1026) directly if you want the eligibility criteria in full.

Side effects: overlapping profiles, individual responses

Both medicines produce a broadly GI-led side-effect profile, nausea, loose stools, constipation, indigestion, and fatigue are the most reported symptoms with either. These are typically most pronounced after a new dose and often settle within a week or two as the body adjusts. Starting at the lowest dose and titrating slowly is specifically designed to reduce this burden; the starter dose is there to help your system adapt, not to produce therapeutic weight loss.

That said, there are individual differences in tolerability that a prescriber factors in. People who have tried one medicine and not tolerated it may respond differently to the other. Neither medicine is recommended during pregnancy, while breastfeeding, or when trying to conceive, and neither is licensed for use under 18. Our team at the clinical team page explains how our prescribers approach these conversations.

If you want a thorough rundown of what to expect day-to-day, our guide to common side effects of weight loss injections covers both medicines in detail. One practical note worth making: dispose of used pens correctly, a 1-litre sharps container is the standard way to do that at home, keeping used needles safe until your local pharmacy or council collection point takes them.

If you think one of these medicines might suit you, checking your eligibility with our prescribers is the first step, it's a free consultation, reviewed the same day by a qualified clinician, not automated software. You may also find it helpful to read about what the 3 weight loss injections are before your consultation, as well as a broader look at all the weight loss injections available so you can ask informed questions.