Weight loss drugs injections: the plain-English guide

What are the injection drugs for weight loss, and why are there two of them?

Mounjaro and Wegovy are the only medicines licensed specifically for weight management by injection in the UK. They belong to a class sometimes called GLP-1 receptor agonists, though that label is only half the story for Mounjaro. Semaglutide (Wegovy) targets one gut-hormone receptor, GLP-1. Tirzepatide (Mounjaro) targets two: GLP-1 and GIP. That dual action is why tirzepatide is described as the only dual-agonist weight-loss medicine available here.

Both slow how quickly your stomach empties, reduce how hungry you feel between meals, and affect the brain signals that drive eating behaviour. If you want to explore how the mechanisms compare in more depth, our page on what the weight loss injection drugs are covers the receptor biology without the jargon.

The two exist because they were developed independently, approved on different timelines, and produce slightly different results. In the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced a greater average weight reduction than semaglutide 2.4mg over 72 weeks in adults with obesity. That does not make one medicine automatically better for any individual, other health factors, tolerability and medical history all shape the right choice.

A third option entered the picture in June 2026: Wegovy is now also available as a daily tablet, the first oral GLP-1 medicine licensed for weight management in the UK. For anyone who prefers to avoid injections altogether, our guide to non-injection weight loss drugs goes through what that means in practice.

Does everyone taking a weight loss injection drug have to use needles?

A question our prescribers hear most weeks, and the short answer is: currently, yes, if you are on Mounjaro. It comes in a KwikPen, a pre-filled device with a very short, fine needle. Wegovy is also a pre-filled pen. Most people who felt anxious about injecting beforehand describe it as less noticeable than they expected; the needle is small and the device is designed to make the process straightforward.

The injection goes into the fatty tissue just beneath the skin, the abdomen, thigh or upper arm are the usual sites. You rotate between them each week to avoid irritating the same spot. The needle goes in at the angle shown in the Patient Information Leaflet; the medicine delivers automatically. Our guide to weight loss injections covers the practical technique in more detail.

If needles remain a firm barrier, the Wegovy tablet (25mg oral semaglutide) is now a licensed alternative. Our page answering whether all weight loss drugs are injections looks honestly at that question, including what the trial data shows for the tablet versus the pen.

Who qualifies for these medicines, and what does the prescriber actually check?

Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition, things like high blood pressure, raised cholesterol, type 2 diabetes or obstructive sleep apnoea. Lower thresholds apply for some ethnic backgrounds under UK clinical guidance. BMI alone does not determine the outcome of a consultation; the prescriber also reviews your medical history, any medicines you already take, and whether there are reasons these treatments are not suitable for you.

That list of reasons matters. Neither medicine is recommended during pregnancy, breastfeeding or when actively trying to conceive. They are not licensed for under-18s. People with a personal or family history of medullary thyroid carcinoma or a condition called MEN2, or those with certain gastrointestinal conditions, require careful discussion before a prescriber would consider either drug. The NHS medicines page for tirzepatide on NHS.uk lists the full contraindications in accessible language.

On the NHS, access to these injection drugs for weight loss is currently restricted to people meeting tighter criteria through specialist services, and waiting lists can be long. Private treatment through a regulated pharmacy is a separate route, subject to the same clinical rules but without the NHS waiting time. You can read more about the differences on our weight loss injections overview page.

What side effects should you realistically expect from weight loss injection drugs?

The most common side effects are gastrointestinal, and the pattern is consistent across both medicines: nausea, loose stools, constipation, indigestion, and sometimes vomiting, particularly in the early weeks or after a dose increase. For most people these settle within a few days to two weeks as the body adjusts. Starting at a low dose and increasing gradually is partly designed to reduce how noticeable they are.

Other reported effects include fatigue, headache, dizziness and mild reactions at the injection site. More serious but less common risks include acute pancreatitis; the MHRA issued a Drug Safety Update on this in January 2026, advising that severe and persistent stomach pain (especially if it radiates to the back) needs prompt medical attention. Gallbladder problems have also been reported. Any suspected side effect, including ones not listed in the leaflet, can be reported directly to the MHRA's Yellow Card scheme.

Cost is worth thinking about too, separately from tolerability. If you are weighing up what treatment involves financially, our breakdown of weight loss injection costs sets out what private prices typically cover and what to check before committing. Speak to our prescribers if you want to understand which option fits your health picture best, that conversation is free, and it is the right starting point.