01 What is Tirzepatide?
In plain English.
Tirzepatide is a lab-made peptide that mimics two natural gut hormones at the same time: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). It's given as a once-weekly injection under the skin and is the active ingredient in two Eli Lilly products: Mounjaro for type 2 diabetes and Zepbound for chronic weight management.
It's the first medicine of its kind, a single molecule that activates both incretin receptors. That dual action is the headline feature, and it's what separates tirzepatide from semaglutide (Ozempic, Wegovy), which acts only on GLP-1.
02 How it works
The simple version, then the science.
Tirzepatide imitates two hormones your gut releases after a meal. Together they tell the pancreas to release insulin when blood sugar is high, slow down stomach emptying so you feel full for longer, and dial down appetite signals in the brain. The result is better blood-sugar control and, for many people, substantial weight loss.
Go deeper · the proposed mechanism
Tirzepatide is a 39-amino-acid synthetic peptide that binds and activates both the GIP receptor and the GLP-1 receptor, an unusual pharmacology for a single molecule. A C20 fatty-diacid moiety binds albumin, extending the half-life to roughly five days and enabling once-weekly dosing. The dual mechanism is hypothesised to drive the additional weight loss seen versus GLP-1-only agents, though the relative contribution of each receptor in humans is still being characterised.
03 What it's used for
Each use graded by how strong the evidence actually is.
- ApprovedType 2 diabetes (glycaemic control)FDA-approved 2022 and EMA-approved 2022 as Mounjaro, as an adjunct to diet and exercise to improve blood-sugar control in adults with type 2 diabetes. The SURPASS programme demonstrated superior A1c reduction versus comparators across five phase-3 trials.
- ApprovedChronic weight managementFDA-approved 2023 as Zepbound and authorised in the UK/EU under Mounjaro, for adults with obesity (BMI ≥30) or overweight with a weight-related comorbidity. SURMOUNT-1 showed mean weight reductions up to 22.5% at 72 weeks.
- ApprovedObstructive sleep apnoea with obesityFDA-approved in 2024 (as Zepbound) for moderate-to-severe OSA in adults with obesity, on the basis of the SURMOUNT-OSA trials, which showed clinically meaningful reductions in apnoea-hypopnoea index.
- StrongHFpEF with obesityThe SUMMIT trial reported lower rates of cardiovascular death or worsening heart failure and improved health status in patients with heart failure with preserved ejection fraction and obesity. Not a labelled indication everywhere, check current local labelling.
04 What the evidence says
The evidence base is among the strongest of any peptide medicine on the market. The SURPASS programme tested tirzepatide in type 2 diabetes against placebo, insulin and other glucose-lowering drugs; the SURMOUNT programme tested it for weight management. The single most-cited finding is from SURPASS-2, a head-to-head trial against semaglutide 1 mg: tirzepatide produced larger A1c reductions and roughly double the weight loss at the 15 mg dose. SURMOUNT-1 reported mean weight loss of 16.0%, 21.4% and 22.5% across the 5/10/15 mg doses at 72 weeks, versus 2.4% on placebo. More recent trials extend the picture into HFpEF with obesity (SUMMIT) and obstructive sleep apnoea (SURMOUNT-OSA), both with positive primary endpoints. Tirzepatide is therefore one of very few peptides where popular interest and rigorous trial evidence point in the same direction, though side effects, cost and long-term safety beyond a few years all remain live questions.
05 Dosing & administration
Reported in the literature, information not advice.
Tirzepatide is prescribed as a once-weekly subcutaneous injection (abdomen, thigh, or upper arm). Labelled starting dose is 2.5 mg per week, titrated upward in 2.5 mg increments at intervals of at least four weeks, with maintenance doses of 5, 10 or 15 mg per week depending on indication and tolerability. Exact titration depends on whether the indication is diabetes or weight management, this is informational only, not a directive. Anyone considering treatment should be assessed and prescribed by a qualified clinician.
06 Side effects & safety
The most common side effects are gastrointestinal: nausea, diarrhoea, constipation, vomiting and reduced appetite, usually mildest during dose titration and easing over time. Serious but less common risks flagged on the FDA label include pancreatitis, gallbladder disease, acute kidney injury (often via dehydration), and severe hypersensitivity reactions. Tirzepatide carries a boxed warning for thyroid C-cell tumours based on rodent data and is contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN-2 syndrome. It is not recommended in pregnancy. Long-term safety beyond a few years is still being characterised.
07 Where to buy (research use only)
Vetted on quality and transparency, not an endorsement to use.
08 Legal & regulatory status
- UKLicensed by the MHRA as Mounjaro for type 2 diabetes and weight management. Prescription-only (POM).
- USFDA-approved as Mounjaro (diabetes, 2022) and Zepbound (weight management, 2023; OSA, 2024). Prescription only.
- EUEMA-approved as Mounjaro since September 2022 for type 2 diabetes and for chronic weight management.
- SportNot on the WADA Prohibited List. From 1 January 2026, tirzepatide is on the WADA Monitoring Program (not banned) to track possible misuse in and out of competition.
09 Clinical studies & research
Primary sources. Read the science yourself.