Tirzepatide is the active ingredient in Mounjaro (and Zepbound in the US). It's a synthetic peptide developed by Eli Lilly that represents a new approach to treating obesity and Type 2 diabetes — one that targets two hormonal pathways simultaneously rather than one.
The dual agonist concept
Most GLP-1 medications — including semaglutide (Wegovy/Ozempic) and liraglutide (Saxenda) — activate a single receptor: the GLP-1 receptor. Tirzepatide is different. It's a dual GIP/GLP-1 receptor agonist, meaning it activates two receptors:1
- GLP-1 receptors: Responsible for appetite suppression, slowed gastric emptying, and improved insulin response — the same pathway targeted by Wegovy and Saxenda
- GIP receptors: GIP (glucose-dependent insulinotropic polypeptide) is another incretin hormone released after eating. It enhances insulin secretion and appears to play a role in fat metabolism, energy expenditure, and how the body processes and stores fat
The theory behind tirzepatide is that activating both pathways simultaneously produces additive or synergistic effects — greater appetite suppression, better metabolic improvements, and ultimately more weight loss than targeting GLP-1 alone. The clinical trial data supports this theory.
What the trials showed
Tirzepatide has been tested in two major trial programmes:234
SURPASS programme (Type 2 diabetes): Demonstrated superior blood sugar control compared to existing treatments, with substantial weight loss as a co-benefit.
SURMOUNT programme (obesity/weight management):
- SURMOUNT-1: 2,539 adults with obesity. Average weight loss of 22.5% at the 15mg dose over 72 weeks — the highest ever recorded for a non-surgical intervention at the time
- SURMOUNT-5: First head-to-head comparison against semaglutide. Tirzepatide achieved 20.2% weight loss vs 13.7% for semaglutide 2.4mg over 72 weeks. Tirzepatide was also associated with greater waist circumference reduction (18.4cm vs 13.0cm) and more people achieving 25%+ weight loss (31.6% vs 16.1%)
How tirzepatide is structured
Tirzepatide is a 39-amino-acid peptide that was engineered to activate both GIP and GLP-1 receptors. It was designed with a C20 fatty diacid moiety that allows it to bind to albumin (a protein in blood), which extends its half-life to approximately 5 days — long enough for once-weekly dosing. It has a roughly five-fold higher affinity for the GIP receptor than the GLP-1 receptor, which distinguishes it pharmacologically from pure GLP-1 agonists.1
UK regulatory status
In the UK, tirzepatide is marketed exclusively as Mounjaro. It is licensed by the MHRA for both Type 2 diabetes and weight management in adults. NICE approved it for obesity treatment in December 2024 (TA1026), and it is now available on the NHS through a phased rollout, as well as through private prescriptions.5
In the US, tirzepatide is sold as Mounjaro for diabetes and as Zepbound for weight management — two brand names for the same drug. For more on this, see our article on Mounjaro vs Zepbound.
What's next for tirzepatide?
Eli Lilly is conducting additional trials to explore tirzepatide's effects on cardiovascular outcomes (SURPASS-CVOT), heart failure, sleep apnoea (SURMOUNT-OSA), and kidney disease. These could expand the range of conditions for which tirzepatide is prescribed and further differentiate it from semaglutide, which already has proven cardiovascular benefits from the SELECT trial.
For a complete guide to Mounjaro including dosing, side effects, cost and eligibility, see our Mounjaro medication page.