Who Developed Tirzepatide — and How the Medicine Came to Be

Step 1, the scientific hypothesis: could two receptors be better than one?

GLP-1 receptor agonists had already demonstrated meaningful weight loss in clinical trials by the early 2010s. Lilly's researchers asked a different question: what would happen if a single molecule activated the GIP receptor alongside GLP-1?

GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are both incretin hormones released from the gut after eating. Each plays a role in appetite regulation and blood-glucose control, but they act through separate pathways. The hypothesis was that engaging both simultaneously might produce effects that neither receptor agonist could achieve alone, more appetite suppression, slower gastric emptying, and a greater overall reduction in caloric intake.

Building a single molecule that binds meaningfully to two distinct receptors without sacrificing potency at either is technically difficult. Lilly's medicinal chemists worked through iterative design cycles across the 2000s and early 2010s, modifying peptide structures until they had a compound with acceptable receptor binding, metabolic stability, and a pharmacokinetic profile suited to once-weekly dosing. Tirzepatide was that compound. For a fuller account of what Mounjaro was originally developed to treat, including its type 2 diabetes origins, our dedicated page covers the regulatory background.

Step 2, clinical trials: the SURPASS and SURMOUNT programmes

Lilly ran two large trial programmes in parallel. SURPASS examined tirzepatide in adults with type 2 diabetes across multiple comparator arms, establishing its glucose-lowering profile. SURMOUNT focused on weight management in adults with obesity or overweight.

SURMOUNT-1 is the trial most cited in UK regulatory discussions. It randomised 2,539 adults without diabetes, comparing tirzepatide at 5mg, 10mg, and 15mg doses against placebo over 72 weeks. The 15mg arm produced an average body-weight reduction of around 20–21%, a result that drew significant attention from regulators and clinicians alike. The paper was published in the New England Journal of Medicine in 2022. A question our prescribers hear most weeks is whether those headline trial figures represent a typical experience, the honest answer is that they are averages across thousands of participants following specific dietary and activity protocols, and individual outcomes vary considerably.

SURMOUNT-5, published in the New England Journal of Medicine in 2025, was an open-label head-to-head against semaglutide 2.4mg over 72 weeks in 751 adults with obesity. Tirzepatide produced greater average weight reduction in that comparison. How tirzepatide compares with semaglutide in practical terms is a decision that rests on your individual medical history, not trial averages alone.

Step 3, regulatory approvals: the path to a UK licence

The MHRA granted UK marketing authorisation for tirzepatide (Mounjaro) for weight management following review of the SURMOUNT evidence package. The licence covers adults with a BMI of 30 or above, or 27 or above alongside at least one weight-related condition such as hypertension, dyslipidaemia, or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance.

Mounjaro carries a Black Triangle (▼) designation, meaning the MHRA continues to collect additional safety data beyond the pre-approval programme. NICE appraised tirzepatide in 2024, publishing guidance (TA1026) in December of that year, which recommended it for NHS use in adults with a BMI of 35 or above plus at least one weight-related comorbidity, with phased rollout criteria. You can read NICE's full tirzepatide appraisal (TA1026) on the NICE website. The background to that NHS appraisal, and who currently qualifies, is covered in detail on our tirzepatide overview page.

Eli Lilly markets the same molecule under the name Zepbound in the United States, where it received FDA approval in late 2023. Zepbound does not exist as a UK product; it is not available from any UK pharmacy, including ours. Anyone searching for Zepbound in the UK is looking for tirzepatide, sold here exclusively as Mounjaro.

What Lilly's development history means for people using Mounjaro today

Understanding who developed tirzepatide matters beyond academic curiosity. Eli Lilly's status as the authorised manufacturer means Mounjaro sourced through the licensed UK supply chain is the identical product tested in the SURMOUNT trials, not a reproduction, not a compounded version. That matters because counterfeit tirzepatide pens have been seized in the UK; the MHRA has warned publicly about fake weight-loss pens circulating online, including an illicit manufacturing site producing counterfeit tirzepatide discovered in 2025.

The cost of genuine Mounjaro through a private pharmacy has shifted since Lilly adjusted UK list prices in September 2025, with typical private costs varying by dose and provider. Context on what Mounjaro costs in the UK is available on our pricing page. At nume, medicines are sourced through Lilly's authorised UK supply chain and dispensed from our GPhC-registered pharmacy (registration 9012878, verifiable at the GPhC register). Every prescription is reviewed by a named GPhC-registered Independent Prescriber before anything is dispensed.

If you want to explore whether tirzepatide is clinically suitable for you, the first step is a free consultation with our prescribers. Check your eligibility on our treatment page.