What Are Tirzepatides — and Why Does the Mechanism Matter?

You've heard it described as a 'jab', here's what's actually in the pen

Picture the pen arriving in plain, unbranded packaging, tracked by DPD right to your door. Inside is a Mounjaro KwikPen: a pre-filled device containing tirzepatide in a clear solution, ready to inject once a week under the skin of your abdomen, thigh or upper arm. Each pen holds four weekly doses. The pen format is the delivery vehicle; tirzepatide is the medicine doing the work.

Tirzepatide belongs to a class sometimes called incretin mimetics. Incretins are hormones the gut releases after eating, they signal fullness, prompt the pancreas and slow the rate at which food leaves your stomach. Tirzepatide is a synthetic molecule engineered to mimic and amplify two of these signals at once: glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). Most people find that within a few weeks food simply becomes less compelling, portions feel adequate sooner, and snacking between meals drops off naturally rather than through willpower. If you are new to this medicine and want to understand what tirzepatide is and how it works, that page is a useful place to start before reading on. You can read the full clinical profile on the tirzepatide overview page.

Because tirzepatide targets two pathways rather than one, it is in a distinct category from semaglutide-based medicines, which act on GLP-1 alone. Whether that makes it the right choice for a specific person depends on their health picture, a prescriber decides, not a headline.

What the SURMOUNT trials tell us, and what they don't

The most-cited evidence comes from SURMOUNT-1, a 72-week randomised trial published in the New England Journal of Medicine. Around 2,500 adults with obesity and no type 2 diabetes took part. Those on 15mg of tirzepatide lost an average of roughly 20–21% of their starting body weight; participants at lower maintenance doses also lost substantial weight, though less on average. NICE reviewed this evidence as part of its appraisal of tirzepatide (TA1026), published in December 2024, and concluded the medicine represents a clinically meaningful option for eligible adults.

What the trials cannot tell you is what will happen in your specific case. Averages conceal a wide range of individual responses. Factors including starting weight, metabolic health, whether a person can tolerate dose increases, and adherence to lifestyle changes all play a role. Trial participants also received structured support alongside the medicine, something worth bearing in mind when comparing results to real-world use.

There is a separate but related question about tirzepatide's composition, if you are curious about what the medicine actually contains at a molecular level, the ingredient breakdown page covers the formulation in more detail.

Who can be prescribed tirzepatide in the UK (and how the thresholds work

Tirzepatide's UK licence covers adults with a BMI of 30 or above, or a BMI of 27 or above where at least one weight-related health condition is present) things like high blood pressure, type 2 diabetes, high cholesterol or obstructive sleep apnoea. For some ethnic backgrounds, UK guidance applies thresholds that are 2.5 kg/m² lower than the standard figures, reflecting differences in how excess weight affects metabolic health. The NHS tirzepatide page summarises the licensed indications in accessible language.

NICE's recommendation under TA1026 sets a higher bar for NHS treatment (BMI of 35 or above plus at least one weight-related comorbidity) and the NHS is rolling out access in phased cohorts. Private routes through a regulated pharmacy do not require NHS referral, but the prescriber still conducts a full clinical assessment. BMI meeting a threshold on paper is the starting point, not the finish line. Things like medication history, existing conditions and any contraindications all feed into the decision.

Treatment starts at 2.5mg (a dose chosen to let the body adjust rather than to produce weight loss) and is titrated upward by the prescriber, typically in four-week steps. Telling someone which dose they should be on or when to change is the prescriber's role, not something to self-manage. For a sense of what private treatment costs, the Mounjaro price comparison page explains what private pricing typically includes and why the cheapest listing is rarely the right measure for a prescription medicine.

A common source of confusion: tirzepatide, Mounjaro and everything else

A question our prescribers hear fairly often is whether tirzepatide and Mounjaro are the same thing. They are, Mounjaro is simply the brand name Eli Lilly chose for their tirzepatide pen. The dedicated page on this goes through the distinction, including why the medicine might be referred to differently in clinical notes versus prescription labels.

Tirzepatide is also sometimes grouped with semaglutide (the ingredient in Wegovy and Ozempic) under the general label of GLP-1 medicines, though that shorthand is slightly loose, tirzepatide is technically a dual GIP/GLP-1 agonist. If you have come across references to peptide-based weight-loss medicines and want to understand where tirzepatide fits, the page on tirzepatide as a peptide is the clearest starting point.

Tirzepatide carries a Black Triangle (▼) designation from the MHRA, meaning it is under additional monitoring as a relatively new medicine. That is standard for recently approved medicines and does not signal unusual danger, it means regulators actively collect real-world safety data. If you notice a side effect worth flagging, the MHRA's Yellow Card scheme is open to patients and healthcare professionals alike.

If you would like to know whether you might be suitable for treatment, you can check your eligibility through a free consultation, reviewed the same day by a GPhC-registered prescriber, never by an algorithm.