The three weight loss injections available in the UK — and what sets them apart

The misconception worth correcting first: these are not the same medicine in different pens

Search results and social media often blur the three injections together, as though they're interchangeable branded versions of one drug. If you want a clear answer to what the weight loss injections actually are, it helps to understand that they are not interchangeable at all. Tirzepatide (Mounjaro) is the only dual agonist in this group, activating both the GIP and GLP-1 receptors. Semaglutide (Wegovy) and liraglutide (Saxenda) are both GLP-1 receptor agonists, but they differ in their molecular structure, dosing schedule, and the depth of evidence behind them.

The practical effect of that difference shows up in the trial data. In SURMOUNT-1, tirzepatide at its highest dose produced average weight reductions of around 20–21% over 72 weeks, the strongest results of any licensed injection in this class. Semaglutide at 2.4 mg produced around 15% average reduction over 68 weeks in the STEP 1 trial, published in the New England Journal of Medicine. Liraglutide, an older medicine taken daily rather than weekly, typically produces more modest reductions and is less commonly initiated today.

None of this means one injection is universally right. Which is appropriate depends on your health history, your current medicines, and a clinical conversation. The headline percentages describe trial averages across thousands of participants, your experience will depend on individual factors a prescriber weighs up during assessment. You can read a broader introduction to how weight loss injections work if you want more context before going further.

What actually distinguishes tirzepatide, semaglutide and liraglutide in practice

Tirzepatide (Mounjaro) comes as a pre-filled KwikPen delivering a once-weekly subcutaneous injection. UK strengths run from 2.5 mg up to 15 mg, and treatment always starts at 2.5 mg, that first dose is there to let your system adjust, not to produce weight loss. The pen is refrigerated until use. NICE recommended tirzepatide for weight management in December 2024 (TA1026), and it now holds a UK licence for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition.

Semaglutide (Wegovy) is also once-weekly and subcutaneous. The standard maintenance dose was 2.4 mg, though the MHRA approved a higher 7.2 mg dose in early 2026 following trial data showing around 20.7% average weight loss at that strength. A dedicated single-dose 7.2 mg pen was approved on 14 April 2026. Wegovy also carries a UK licence for BMI 30 and above, or 27 and above with a weight-related condition.

Liraglutide (Saxenda) requires a daily injection rather than weekly, which some people find harder to maintain. It is licensed in the UK for weight management but produces smaller average reductions than the two above. It is worth knowing that Saxenda is not dispensed by our pharmacy, we focus on the two medicines with the most robust recent evidence.

One thing that is consistent across all three: injection sites. Abdomen, thigh, and upper arm are all used, and rotating sites matters. For general questions about self-injection technique and disposal, our FAQ page covers the basics.

Who these injections are licensed for, and why BMI alone is not the whole answer

The private licensing criteria for both tirzepatide and semaglutide are similar on paper: a BMI of 30 or above, or 27 and above alongside a condition such as high blood pressure, type 2 diabetes, obstructive sleep apnoea, dyslipidaemia, or cardiovascular disease. UK guidance also recognises that lower BMI thresholds are appropriate for some ethnic backgrounds. But a licence threshold is not a guarantee of a prescription.

A prescriber considers the complete picture: your current medicines, any history of pancreatitis or certain thyroid conditions, whether you are pregnant or breastfeeding, and your weight-management goals. These injections are not licensed for use in under-18s, and they are not recommended during pregnancy or while trying to conceive. The NHS medicines page for tirzepatide and its semaglutide equivalent set out the contraindications clearly and are worth reading before your first consultation.

If you are wondering how private treatment through a regulated service compares with NHS access, the page on newer injections for weight loss covers the NHS rollout timeline and waiting-list context in more detail. Costs are another common question, a straightforward breakdown of what private treatment typically includes is available on our weight loss injection prices page.

Side effects to be aware of across all three injections

The side-effect profile of all three injections is dominated by gastrointestinal symptoms: nausea, loose stools, constipation, reflux, and burping are the most common. These typically land hardest in the days after a dose increase and settle within one to two weeks for most people. Fatigue and headache are also reported, particularly early on.

One side effect that warrants specific mention is acute pancreatitis. In January 2026, the MHRA issued a Drug Safety Update for GLP-1 medicines highlighting acute pancreatitis as an infrequent but potentially serious risk. Severe stomach pain that reaches through to the back (with or without vomiting) is a reason to get medical help promptly, not to wait and see. Any suspected side effect, whether serious or not, can be reported directly through the MHRA Yellow Card scheme.

Tirzepatide carries a specific note for women using oral contraceptives: because it slows gastric emptying, pill absorption may be reduced during the first four weeks of treatment and for four weeks after each dose increase. Adding a non-oral method during those windows is the practical advice, and our prescribers discuss this at consultation for anyone it applies to. If you have questions before starting, our support team is available seven days a week. You may also find it useful to read about what the 3 weight loss injections are or, if you are choosing between tirzepatide and semaglutide specifically, the page covering what the 2 weight loss injections are goes into that comparison in more detail.

When you order through nume and treatment is approved, your medication arrives by DPD the next working day in plain, unbranded packaging, the tracking link comes to your phone so you know exactly when to expect it.