What Are the Injectable Weight Loss Drugs and How Do They Work?

You've heard about 'weight loss jabs', here's what's actually in those pens

Imagine you've seen headlines about 'the jab that makes people eat less' and you're not entirely sure whether they're talking about one drug or several. That confusion is understandable. The shorthand in press coverage compresses what is actually a small, distinct group of prescription medicines into a single vague category.

In the UK, two injectable medicines hold a licence specifically for weight management in adults: tirzepatide, sold as Mounjaro, made by Eli Lilly; and semaglutide, sold as Wegovy, made by Novo Nordisk. A third name you may have come across, Ozempic, is also semaglutide but it is licensed for type 2 diabetes, not weight loss, and prescribers are not supposed to use it for that purpose. The NHS medicines page for tirzepatide sets out plainly what it is, what it does and what side effects to expect.

What both Mounjaro and Wegovy have in common: they belong to a family of medicines called GLP-1 receptor agonists, they are given as a once-weekly subcutaneous injection into the abdomen, thigh or upper arm, and they work in part by slowing how quickly the stomach empties and by acting on appetite centres in the brain. The result, for most people, is that they feel full sooner and stay full longer. That mechanism is not magic and it is not cosmetic, these are medicines that carry a Black Triangle designation from the MHRA, meaning they are under additional monitoring. Our clinical team keeps up with every safety communication as it comes through.

How the two drugs differ, and why that distinction matters before you pick one

Tirzepatide is the only dual-agonist in this group: it activates both the GIP receptor and the GLP-1 receptor. Semaglutide activates GLP-1 alone. That difference in mechanism is part of the reason the trial evidence shows a gap in average weight loss between the two medicines, though the gap narrows at Wegovy's newer, higher dose. In the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks in adults with obesity who did not have diabetes.

Both treatments start at a low dose (the first pen exists to let your body adjust, not to produce results) and are titrated upward by the prescriber in roughly four-week steps. Mounjaro goes from 2.5mg to a maximum of 15mg across six dose levels. Wegovy's standard maintenance target is 2.4mg weekly, though the MHRA approved a higher 7.2mg dose in early 2026. If you want a full picture of the injectable drugs used for weight loss, including how they compare and what distinguishes one from another, our dedicated guide covers that in detail. There is also now an oral semaglutide tablet licensed for weight loss, which is a separate product and a separate decision; you can read more in our guide to weight loss injections if you want to compare formats first.

Eligibility for both medicines follows broadly the same rule: 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. A prescriber looks at the full picture, not the number alone.

One thing worth doing right now, before you go further: if you are considering an online pharmacy, spend thirty seconds checking it on the GPhC register. A pharmacy not listed there is not a legal dispenser of prescription medicines in the UK.

Side effects, safe use and what the clinical evidence actually says

The most common side effects of both medicines are gastrointestinal: nausea, loose stools or constipation, indigestion and occasionally vomiting. These tend to be most pronounced when starting or after a dose increase, and for most people they ease within a week or two. Fatigue, headache and dizziness are also reported. More serious but less common effects (including acute pancreatitis) led the MHRA to issue a specific Drug Safety Update for GLP-1 medicines in January 2026; severe stomach pain that spreads to the back, with or without vomiting, warrants urgent medical attention. Suspected side effects can be reported at the MHRA's Yellow Card scheme.

Clinical trial evidence, across thousands of adults in the SURMOUNT and STEP programmes, recorded average weight reductions of roughly 15% at 68 weeks for semaglutide 2.4mg and around 20–21% at 72 weeks for tirzepatide 15mg, always alongside a reduced-calorie diet and increased activity. These are averages. Individual responses vary, and neither medicine is suitable for everyone. Women using 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 the pill's absorption may be reduced; the NHS England page on weight management injections covers this and related practical guidance in full.

Neither medicine is licensed for anyone under 18, and neither is recommended during pregnancy, while breastfeeding or when trying to conceive. If you are in any of those situations, that conversation starts with your GP or prescriber.

Getting access: what the NHS route looks like versus the private route

Both Mounjaro and Wegovy are available on the NHS, but eligibility criteria are strict and phased, and waiting lists through specialist services can be long. NICE recommends tirzepatide for adults with a BMI of 35 or above alongside at least one weight-related comorbidity; the NHS rollout is expanding in stages through 2026 and 2027. If you are still getting to grips with what the weight loss injection drugs are actually called, our guide runs through the names, brand names and distinctions in plain language. For people who do not yet meet those criteria, or who would rather not wait, a private prescription from a regulated online pharmacy is the legal alternative. You can get a broader sense of how these options compare on our page exploring whether all weight loss drugs are injectable, which also covers oral alternatives.

If you are thinking about the cost, what weight loss injections actually cost in the UK depends on the dose, the provider and what is included in the price. A legitimate private price covers the prescription itself, the clinical assessment and, at a properly run pharmacy, ongoing aftercare. At nume, every repeat order goes through a fresh clinical review by a GPhC-registered prescriber (not a checkbox system) before anything is dispatched. Our frequently asked questions page covers the process in more detail if you want it before booking anything.

These medicines are not a quick fix and they are not cosmetic. Used correctly, with clinical oversight and lifestyle support, they are the most effective weight-management medicines the UK has licensed to date. Whether one of them is right for you is a question answered by assessment, not by a headline.

If you'd like to explore your options with a clinician, speak to our prescribers through a free consultation, reviewed the same working day by a named GPhC-registered prescriber.