All the Weight Loss Injections Explained: What They Are, How They Differ, and What to Expect

Which injections are actually licensed for weight loss in the UK right now?

The short answer: two. Mounjaro (tirzepatide), made by Eli Lilly, and Wegovy (semaglutide), made by Novo Nordisk. That is the complete list of injections licensed by the MHRA for weight management in adults. If a seller is offering something else as a weight loss injection (a compounded product, an unlicensed import, something described in vague terms) that is a serious red flag worth acting on.

You may have also come across Ozempic, Saxenda, or Rybelsus in your research. Saxenda (liraglutide) was previously used for weight management but is not dispensed by every service. Ozempic is semaglutide in a different dose, licensed exclusively for type 2 diabetes, not for weight loss. Rybelsus is an oral tablet form of semaglutide, again for diabetes. The names circulating online can be genuinely confusing, and that confusion is worth clearing up before anyone hands over money or rolls up a sleeve.

Both Mounjaro and Wegovy are Black Triangle (▼) medicines, meaning the MHRA collects additional safety data on them. That is standard for newer medicines and does not mean they are experimental; it means the monitoring is thorough. The NHS medicines page for tirzepatide covers this clearly.

How do these injections work, and are they the same?

They are not the same, and the distinction matters clinically. Wegovy activates a single gut-hormone receptor: the GLP-1 receptor. This slows gastric emptying, increases feelings of fullness after eating, and reduces appetite. Mounjaro activates two receptors (GLP-1 and GIP) making it the only dual-agonist weight loss medicine licensed in the UK. Whether that dual action translates into a meaningful difference for a given patient is a clinical judgement, but the trial data does suggest a difference at population level.

In the SURMOUNT-1 trial (2,539 participants, 72 weeks), tirzepatide produced an average body-weight reduction of around 20–21% at the highest dose. The STEP 1 trial of semaglutide 2.4mg showed an average reduction of approximately 15% over 68 weeks. More recently, the SURMOUNT-5 head-to-head trial (published in the New England Journal of Medicine in 2025) compared tirzepatide directly against semaglutide 2.4mg over 72 weeks in adults with obesity and found greater average weight loss with tirzepatide. That said, many people do extremely well on Wegovy, and a higher dose of semaglutide (7.2mg, approved by the MHRA in January 2026) narrows the gap further.

If you want to understand whether all weight loss injections work the same way, the mechanism page covers the science in more depth. The practical upshot here: these are related but distinct medicines, and the right one for you is not determined by a search engine.

What can you expect in terms of side effects and how they're managed?

Both injections share a broadly similar side-effect profile because both act on the gut. Nausea is the most commonly reported effect, particularly when starting or moving up to a higher dose. Diarrhoea, constipation, indigestion, burping, and fatigue also appear in trial data for both medicines. For most people these effects are worst in the first week or two after a dose change and then settle.

There are some differences worth knowing. For tirzepatide specifically, the MHRA advises that women using oral contraceptives should add a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because the pill's absorption may be reduced by slowed gastric emptying. NHS guidance does not flag the same concern for semaglutide, though the principles of discussing any medicine interaction with a prescriber still apply. If you take other medicines (including for conditions like arthritis) it is worth reading about how weight loss injections interact with treatments like methotrexate before starting.

On pancreatitis: the MHRA issued specific guidance in January 2026 highlighting acute pancreatitis as an infrequent but serious risk with GLP-1 medicines. Severe stomach pain that reaches through to the back, with or without vomiting, needs urgent medical attention. Report any suspected side effects (from either injection) via the MHRA Yellow Card scheme.

Who can access these injections, and how does the process work?

Both injections are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related condition such as high blood pressure, type 2 diabetes, high cholesterol, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. These are the licence parameters; a prescriber also considers your full health picture, any contraindications, and your current medicines before approving treatment.

On the NHS, access follows NICE criteria and phased rollout timelines, waiting lists for NHS weight loss injections are currently long in most areas. Private treatment through a regulated pharmacy is the alternative for people who don't meet the NHS criteria yet, or who cannot wait. For context on what private treatment costs and what that price should include, the weight loss injection pricing guide is a straightforward place to start.

It is reasonable to feel a bit overwhelmed at this stage, there's a lot of noise around these medicines online, and not all of it is accurate. If you'd like a prescriber to look at your specific situation, speak to our prescribers through a free consultation and get a clinical answer rather than a general one. The starting point is our weight loss treatments page.