How tirzepatide's GLP-1 and GIP mechanism differs from every other weight-loss injection

What GLP-1 and GIP actually do, and why targeting both matters

GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide) are both incretin hormones, chemical signals released by the gut after a meal. GLP-1 slows stomach emptying, suppresses glucagon, and tells the brain that the body has eaten enough. GIP works alongside it, influencing how fat tissue handles energy and amplifying the insulin response to food. They reinforce each other.

The distinction from earlier GLP-1-only medicines is meaningful. Semaglutide, for example, binds only the GLP-1 receptor. Tirzepatide binds both, and does so with high potency at each site. The combined signal appears to suppress appetite more durably than either hormone manages on its own, which is reflected in the trial results. The relationship between tirzepatide's GLP-1 and GIP receptors is explored further if you want to go deeper on the receptor biology.

There is also a metabolic dimension. GIP receptors are found on fat cells as well as in the pancreas, and activating them seems to improve how the body handles lipids, which may partly explain why the cardiometabolic markers (blood pressure, waist circumference, triglycerides) also shifted favourably in the SURMOUNT programme, not just body weight. These are still active research areas, the NHS's medicines page on tirzepatide notes the Black Triangle status precisely because longer-term data continue to be collected.

What the SURMOUNT-1 evidence actually showed

SURMOUNT-1 was a 72-week randomised controlled trial of 2,539 adults with obesity but without type 2 diabetes, published in the New England Journal of Medicine. Participants on 15 mg tirzepatide lost an average of around 20–21% of body weight. Roughly 37% of them lost 25% or more. Those are figures from a controlled trial setting with lifestyle support, real-world results vary, which is why the evidence is presented as averages, not guarantees.

A question our prescribers hear most weeks is whether tirzepatide's results are really that different from semaglutide's, or whether the headline numbers are just marketing. The SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, offers the clearest answer so far: tirzepatide produced greater average weight reduction than semaglutide 2.4 mg over 72 weeks in adults with obesity and no diabetes. The gap narrows as semaglutide's dose rises (Wegovy's 7.2 mg maintenance dose is now approved in the UK) but at comparable doses the dual-receptor mechanism does appear to translate into a measurable difference. Whether Mounjaro counts as a GLP-1 drug is a fair question that the dual-agonist classification sometimes muddles.

NICE reviewed this evidence for TA1026, published in December 2024 and updated in September 2025, and recommended tirzepatide for NHS use in eligible adults, a decision that reflected both clinical efficacy and cost-effectiveness modelling.

How the dual mechanism shapes the side-effect profile and what to expect

Because tirzepatide engages GLP-1 receptors, it shares the GI side-effect profile common to that drug class: nausea, vomiting, diarrhoea, constipation, indigestion, and sometimes burping or fatigue, especially in the early weeks or after a dose step up. These typically settle once the body adjusts, the graduated titration schedule exists precisely to give the gut time to adapt.

The GIP receptor adds a layer that is still being characterised. Current evidence does not show a meaningfully different safety signal compared with GLP-1-only medicines at the population level, but it is part of why the Black Triangle designation is in place. The NHS tirzepatide medicines page lists the full side-effect profile and the signs that should prompt urgent medical attention, including severe persistent stomach pain that may radiate to the back, which can indicate pancreatitis and needs same-day assessment.

Storage follows refrigeration requirements (2–8°C); the exact room-temperature window is set out in the Patient Information Leaflet, and that is the definitive source to follow. For a broader look at the practical side of treatment, the tirzepatide overview covers eligibility, the titration schedule and what to discuss with a prescriber.

What this mechanism means for who the medicine suits

Tirzepatide is licensed in the UK for adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, dyslipidaemia, type 2 diabetes or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. A BMI that meets the threshold is the starting point; suitability depends on the prescriber's full assessment of someone's history and other medicines.

The GIP component has a specific relevance to people with type 2 diabetes, tirzepatide holds a UK licence for that indication too, and its dual-pathway effect on insulin secretion and blood-sugar control is explained in detail in the Mounjaro mechanism of action page. For weight management without diabetes, the mechanism still operates, and the SURMOUNT-1 cohort (adults without diabetes) is where the headline weight-loss figures come from.

For context on what treatment costs as a private patient, the weight-loss treatments page sets out what is included in a single transparent price. If you have questions about how the mechanism relates to your own situation, the clinical team at nume's prescribing lead (sorry) at our pharmacy reviews every consultation personally. Start your free consultation whenever you are ready.