What are the weight loss injection drugs available in the UK?

The two drugs and how they differ in the way they act

Tirzepatide, sold as Mounjaro, is made by Eli Lilly and works on two gut-hormone receptors simultaneously: GIP and GLP-1. That dual action makes it the only weight-loss injection of its kind currently licensed in the UK. Semaglutide, sold as Wegovy, is made by Novo Nordisk and activates the GLP-1 receptor alone. Both slow gastric emptying and reduce appetite, but they take slightly different biological routes to get there, and if you are new to this area our page on what the weight loss injections actually are is a good place to start.

In practice, both come as pre-filled pens injected once a week under the skin of the abdomen, thigh or upper arm. Treatment always starts at a low tolerability dose (2.5mg for tirzepatide, 0.25mg for semaglutide) and a prescriber steps the dose up gradually over several months. The NHS patient information pages for tirzepatide and semaglutide set out the full schedules clearly.

A head-to-head trial published in 2025 (SURMOUNT-5, New England Journal of Medicine) found that tirzepatide produced greater average weight reduction than semaglutide 2.4mg over 72 weeks among adults with obesity who did not have diabetes. That gap narrows at semaglutide's newer 7.2mg dose, approved by the MHRA in 2026. Which drug is appropriate for any individual is a clinical question, it depends on health history, other medicines, and how a person's body responds. You can read more about how these drugs compare in detail if you want to explore the evidence further.

Who these medicines are licensed for, and when the prescriber decides against them

Both tirzepatide and semaglutide share similar eligibility thresholds for private prescription in the UK. The licensed criteria cover adults with a BMI of 30 or above, or a BMI of 27 or above alongside at least one weight-related health condition such as high blood pressure, high cholesterol, type 2 diabetes, prediabetes or obstructive sleep apnoea. Lower BMI thresholds can apply for some ethnic backgrounds under UK guidance.

A BMI figure alone does not settle suitability. Prescribers also weigh up whether a person has a history of certain conditions (including pancreatitis, medullary thyroid carcinoma or multiple endocrine neoplasia type 2) that would make these medicines unsuitable. Neither drug is licensed for under-18s, and both are not recommended during pregnancy, while breastfeeding, or when trying to conceive. Anyone already taking other medicines needs their full list reviewed, because drug interactions matter.

If you are unsure where you stand, our frequently asked questions cover the most common eligibility queries, and our prescribers are available to discuss borderline situations directly.

Side effects worth knowing before you decide

The most common effects are gastrointestinal: nausea, loose stools, constipation, indigestion, and occasionally vomiting. These tend to be most noticeable in the first few weeks after starting or after a dose increase, and most people find they settle as the body adjusts. Starting at a low dose specifically helps manage this early phase.

Less common but more serious effects exist. In January 2026 the MHRA issued a Drug Safety Update highlighting acute pancreatitis as an infrequent but potentially serious risk: seek urgent medical help for severe stomach pain that spreads to the back, with or without vomiting. Gallbladder problems and dehydration from prolonged vomiting or diarrhoea are also flagged in the prescribing guidance. Any suspected side effect can be reported at yellowcard.mhra.gov.uk.

Women using oral contraceptives who start tirzepatide should add a barrier method for the first four weeks of treatment and for four weeks after each dose increase, because tirzepatide's effect on gastric emptying may reduce pill absorption. If you have questions about how either drug interacts with other medicines you take, that conversation belongs in your consultation, it is exactly the kind of detail our prescribers are there to work through with you. If you are still deciding whether an injection is the right format for you, our page on whether all weight loss drugs are injections covers the alternatives available. You can also read about the broader range of injectable options if you want context before booking.

Getting a legitimate prescription, and why the source matters

These are prescription-only medicines. In the UK, the only lawful route is a prescription issued by a registered prescriber following a clinical assessment. That means a qualified clinician reviews your answers, your health history and (for a reputable online pharmacy) verifies your identity and weight before anything is dispensed. There are no shortcuts that preserve that safety margin.

The MHRA has seized large quantities of fake weight-loss pens sold through social media and unverified online sellers; some counterfeit pens have contained insulin. The clearest red flag is any seller offering these medicines without a clinical assessment. Always verify that an online pharmacy holds a GPhC registration, you can check ours (registration 9012878) at the GPhC pharmacy register.

For context on what a private prescription typically costs and what to check before comparing prices, our weight loss overview page explains what a legitimate prescription service includes. If you want to understand how a weight loss injection works in practice, that page covers the self-injection experience in plain language. If you are curious about the names of the weight loss injection drugs and how they relate to the branded pens you may have seen mentioned, that page sets it out clearly. At nume, every consultation is reviewed the same day by a GPhC-registered Independent Prescriber. Order before 12pm on a working day and, once clinically approved, your treatment leaves our pharmacy the same afternoon, delivered free by DPD the next working day, tracked, in plain packaging. Worth checking whether that works for your schedule.