The drug tirzepatide: mechanism, licence and what to expect

Step 1 — understanding what kind of drug tirzepatide is

Tirzepatide belongs to a class of medicines called GIP and GLP-1 receptor agonists. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are hormones the gut releases after eating; among other things, they signal fullness to the brain and slow the rate at which the stomach empties. By mimicking both signals at once, tirzepatide reduces appetite more consistently than medicines acting on GLP-1 alone. The NHS tirzepatide medicine page summarises this mechanism clearly and is worth bookmarking alongside any patient information leaflet.

It is worth being precise about the drug's identity, because a question our prescribers hear most weeks is whether Mounjaro and tirzepatide are different things. They are not: Mounjaro is simply the brand name under which Eli Lilly markets tirzepatide in the UK. The active molecule in every pen is tirzepatide. This page on tirzepatide's drug names covers the full naming picture, including the INN and other international brand names.

Tirzepatide carries a Black Triangle (▼) designation from the MHRA, meaning it is subject to additional monitoring while post-market safety data continues to build. That does not mean it is considered unsafe, Black Triangle status is standard for newer medicines and is reviewed regularly.

Step 2, the dose journey from 2.5 mg to maintenance

Every tirzepatide prescription starts at 2.5 mg. The first pen's job is to let your system adjust to the drug, not to produce meaningful weight loss, the therapeutic effect builds as the dose rises. From there, prescribers typically move the dose up in four-week steps through 5, 7.5, 10 and 12.5 mg, reaching 15 mg if tolerated. The pace of that titration is always a clinical decision, not a fixed rule, so your prescriber may slow it if side effects are noticeable at a given dose.

Each Mounjaro KwikPen holds four weekly doses, so one pen covers a full four-week period. Injections go under the skin of the abdomen, thigh or upper arm; rotating the site each week reduces local skin reactions. Storage is refrigerated at 2–8°C, and there is a limited window during which the pen can be kept at room temperature, the SmPC sets out the exact period, and you should follow that, not a rough estimate. The Mounjaro overview page covers storage and pen use in more practical detail.

If you are thinking about what distinguishes tirzepatide from semaglutide or other GLP-1 drugs, this guide to the type of drug tirzepatide is sets out the pharmacological distinctions clearly.

Step 3, reading the evidence: SURMOUNT-1 and NICE's position

The pivotal trial for tirzepatide's weight-management licence was SURMOUNT-1, which randomised 2,539 adults with obesity and no diabetes to tirzepatide or placebo over 72 weeks. At the 15 mg dose, average body-weight reduction was around 20–21%, with some analyses reaching approximately 22.5%, figures that set a new benchmark for a licensed weight-loss medicine at the time. The trial was published in the New England Journal of Medicine and formed the primary evidence base for NICE's appraisal.

NICE recommended tirzepatide in December 2024 (technology appraisal TA1026) for NHS use in adults with a BMI of 35 or above and at least one qualifying weight-related comorbidity. For certain ethnic backgrounds (including South Asian, Chinese, Middle Eastern, Black African and African-Caribbean) the threshold is 2.5 kg/m² lower. NHS access remains phased and is rolling out gradually through GP practices and specialist services; private treatment through a regulated online pharmacy is the current route for people outside those NHS criteria or who cannot wait for an NHS appointment.

For a full picture of what tirzepatide treatment involves beyond the drug itself, this page on tirzepatide drugs covers the wider treatment context.

Step 4, side effects, eligibility and when a prescriber is essential

The most commonly reported side effects are gastrointestinal: nausea, vomiting, diarrhoea, constipation and reflux tend to be most noticeable after a dose starts or increases, and for many people they ease within a couple of weeks. Fatigue, headache and injection-site reactions are also reported. None of these should be minimised, but nor should they be alarming, they are the pattern seen with this class of medicine, and your prescriber can advise on managing them.

Tirzepatide is not suitable for everyone. It is not licensed for people under 18, and it is not recommended during pregnancy, breastfeeding or when trying to conceive. A history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 rules it out entirely; a personal or family history of pancreatitis requires careful prescriber review. Women taking oral contraceptives should add a barrier method for the first four weeks of treatment and after each dose increase, because the drug's effect on gastric emptying can reduce how reliably the pill is absorbed. The cost context for private treatment is worth understanding too, the Mounjaro cost page sets out what legitimate private pricing typically covers.

If you are already on tirzepatide and have questions about whether it would show on a drugs test, this page on tirzepatide and drug testing addresses that directly. And if you want to understand how tirzepatide fits the broader Mounjaro drug profile, the Mounjaro drug page is a good companion read. When you are ready to find out whether treatment is clinically suitable for you, check your eligibility with our clinical team, consultations are free, and a GPhC-registered prescriber reviews every case the same day.