Tirzepatide class: the dual receptor agonist behind Mounjaro

The misconception: tirzepatide is 'just another GLP-1 drug'

This is probably the question our prescribers hear most weeks, people assume Mounjaro works the same way as Wegovy or Ozempic, just with a different brand name. It does not. Semaglutide and liraglutide act on a single receptor: GLP-1. Tirzepatide activates two: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). The GIP pathway is a separate gut-hormone system involved in fat metabolism and energy balance. Switching one receptor for two is not a minor tweak; it is a fundamentally different pharmacological approach.

The practical consequence shows in the trial data. In SURMOUNT-1, involving 2,539 adults with obesity but without diabetes, participants taking tirzepatide at the 15mg dose achieved an average body-weight reduction of around 20–21% over 72 weeks, figures that had previously belonged only to surgical outcomes. The SURMOUNT-1 paper published in the New England Journal of Medicine remains the primary evidence base for these results. Classifying tirzepatide alongside single-pathway GLP-1 medicines understates what the mechanism actually does.

What GIP and GLP-1 receptors each contribute

GLP-1 receptors sit in the gut, the pancreas and the brain. When activated, they slow the rate at which food leaves the stomach, prompt insulin release in response to a meal, and send fullness signals to the hypothalamus. That is the basis of every GLP-1 medicine currently available. The effect on appetite is real and well-established.

GIP receptors add a separate layer. GIP is secreted by the small intestine in response to dietary fat and carbohydrates. Activating the GIP pathway appears to reinforce the appetite-suppressing effect from a different direction, and may also influence how the body handles fat storage. The precise interplay between the two pathways in tirzepatide lies at the heart of what makes it pharmacologically distinct, and the combined signal appears to produce greater reductions in caloric intake than either pathway alone. You can read more about tirzepatide's pharmacology and licensed uses in our full medicine overview. The NHS medicines page on tirzepatide also provides an accessible summary of how it works and what to expect.

What drug classification means for prescribing and eligibility

In the UK, tirzepatide's drug classification matters for two reasons: who can receive it, and how it is monitored. As a prescription-only medicine, it can only be dispensed following a clinical assessment by a qualified prescriber. The Black Triangle (▼) status means Eli Lilly and the MHRA are actively collecting post-marketing data; patients and clinicians are encouraged to report any suspected side effects via the Yellow Card scheme.

NICE recommended tirzepatide for NHS use in TA1026, published in December 2024, for adults with a BMI of 35 or above alongside at least one weight-related comorbidity. Private eligibility follows the licensed criteria: a BMI of 30 or above, or 27 and above where a weight-related condition such as hypertension, type 2 diabetes or obstructive sleep apnoea is present. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. The classification as a dual agonist does not widen or narrow who qualifies, that is determined by the SmPC criteria and the prescriber's assessment of the full clinical picture. For a detailed breakdown of the Mounjaro drug class and how it compares to adjacent medicines, that page covers the prescribing context in more depth.

If you are considering treatment and want to understand how the pharmacological class translates into a clinical decision, the Mounjaro class of medication page sets out the comparisons clearly. Cost context for private treatment is available on our weight-loss treatments page.

How tirzepatide's class compares to semaglutide in practice

Semaglutide, used in Wegovy, is a GLP-1 receptor agonist, one pathway. Its results are meaningful: the STEP 1 trial reported around 15% average body-weight reduction over 68 weeks at the 2.4mg maintenance dose. If you want to understand how Mounjaro performed against semaglutide directly, SURMOUNT-5 compared the two in adults with obesity and found tirzepatide produced greater average weight loss over 72 weeks, the first robust head-to-head evidence in this space.

What drug class is tirzepatide? Formally, it is categorised in the BNF as a glucose-dependent insulinotropic polypeptide (GIP) and GLP-1 receptor agonist, a subcategory that currently contains only one licensed medicine. That specificity matters: when a prescriber or pharmacist looks up the drug classification, tirzepatide sits in its own bracket rather than sharing one with semaglutide. The tirzepatide drug classification page covers the formal BNF and regulatory categorisation in full detail.

Which medicine suits a given person is a clinical decision, not a comparison-chart outcome. If you would like to explore that with a qualified prescriber, we are here. Speak to our prescribers through a free consultation and get a same-day clinical review.