Tirzepatide: what type of drug is it and how does it work?

Step 1, two receptors, one injection: what makes tirzepatide different from other GLP-1 drugs

Most weight-loss medicines currently available in the UK target a single hormone receptor. Semaglutide (Wegovy), for example, activates the GLP-1 receptor only. Tirzepatide does something additional: it also activates the GIP receptor, which stands for glucose-dependent insulinotropic polypeptide. That dual action is why researchers and clinicians classify it separately from the GLP-1 drugs that came before it, and if you want a clear breakdown of the category it belongs to, our page on what type of drug Mounjaro is explains how it sits within the broader pharmacological landscape.

Both GIP and GLP-1 are incretin hormones, released naturally by the gut after eating. They signal to the brain that food has arrived, prompt the pancreas to release insulin in response to rising glucose, and slow the rate at which the stomach empties. Activating both pathways together appears to produce a stronger effect on appetite suppression than either would achieve alone, which is the rationale behind the dual-agonist design.

The BNF's entry on tirzepatide summarises this mechanism concisely for those who want a clinical-level reference. For a fuller comparison of how tirzepatide sits alongside other medicines, our page on the different types of tirzepatide preparations covers the landscape.

Step 2, the licensed schedule: how tirzepatide moves from a starter dose to a maintenance dose

Tirzepatide is manufactured by Eli Lilly and supplied in the UK as the Mounjaro KwikPen, a pre-filled device containing four weekly doses. The medicine comes in six strengths: 2.5, 5, 7.5, 10, 12.5 and 15 mg.

Treatment begins at 2.5 mg. That opening dose exists to help your system adjust gradually, nausea and digestive discomfort are most common when the medicine is new or when the dose rises, and starting low reduces the likelihood of either. The prescriber then increases the dose in steps, typically every four weeks, up to the highest dose that suits you clinically. Not everyone reaches 15 mg, and that is fine; the dose that works for you is the one your prescriber lands on.

Storage is straightforward: the pen lives in the fridge, ideally on a door shelf, between 2 °C and 8 °C. The SmPC specifies how long a pen can be kept at room temperature before it must be used or discarded, your Patient Information Leaflet is the right reference for that exact window, not a webpage. The NHS medicines page for tirzepatide covers storage, injection technique and common questions in plain English.

If you are curious about what the clinical evidence says about outcomes at different doses, our page on tirzepatide's clinical profile goes into the trial data in detail.

Step 3, what the evidence shows, and what the licence says

Tirzepatide's UK licence covers two indications: weight management in adults with a BMI of 30 or above (or 27 or above with at least one weight-related health condition), and type 2 diabetes. These are separate clinical contexts, and a prescriber will assess which applies to you.

The weight-management evidence comes primarily from the SURMOUNT clinical programme. SURMOUNT-1, which randomised 2,539 adults with obesity but without diabetes across 72 weeks, found average body-weight reductions of around 20 to 21% at the 15 mg dose, figures that placed tirzepatide ahead of any previously licensed weight-loss medicine at the time. The SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, subsequently compared tirzepatide head-to-head with semaglutide 2.4 mg and found greater average weight reduction with tirzepatide.

NICE appraised tirzepatide in December 2024 (Technology Appraisal TA1026) and recommended it for adults with a BMI of at least 35 plus one or more weight-related health conditions. Lower BMI thresholds apply for certain ethnic backgrounds under that guidance. Critically, meeting the NICE criteria does not guarantee NHS access, rollout is phased, and eligibility criteria are narrower than the licensed indications. Private treatment through a registered pharmacy is the route for people who meet the licensed criteria but not the current NHS thresholds, or who prefer not to wait.

For context on what private treatment involves financially, our treatment page sets out what is included in the price. Our Mounjaro drug page covers how the medicine is used specifically under its UK brand name.

Step 4, what being a prescription-only drug means in practice

Tirzepatide's POM classification is not a bureaucratic formality. GLP-1 and dual-agonist medicines interact with other drugs, carry contraindications for certain conditions, require baseline checks, and need ongoing clinical oversight as the dose changes. A prescriber reviews whether the medicine is appropriate for you, not just whether your BMI clears a threshold.

The MHRA has been clear that these medicines are not assessed for quick or cosmetic weight loss. Several enforcement actions have followed, including seizures of counterfeit pens sold without any clinical assessment. The legitimate route is a prescription from a clinician who has reviewed your health information, and the only way to verify that an online pharmacy is operating legally is to check it on the GPhC register. One question that comes up regularly in this context is whether tirzepatide affects standard screening panels, and our page on tirzepatide and drug tests addresses that directly for anyone who needs clarity before starting treatment.

At nume, every consultation is read by a GPhC-registered Independent Prescriber, not processed by software. If you have questions about whether tirzepatide might be appropriate for you, our clinical team is the right place to start. You can also read more about the full range of weight-loss options at our weight-loss treatment overview, or check our frequently asked questions for quick answers on how the process works.

When you are ready, starting a free consultation is straightforward.