Tirzepatide

Tirzepatide is a synthetic peptide that acts on the receptors for two incretin hormones: glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). It carries a fatty-acid modification that lets it circulate for about a week. Eli Lilly first reported it in 2018 under the code LY3298176, describing a molecule designed to test whether adding GIP activity improves on selective GLP-1 drugs such as semaglutide ^[1].

It is sold as two branded medicines: Mounjaro, approved in the US in May 2022 for type 2 diabetes, and Zepbound, approved in the US in November 2023 for chronic weight management and in December 2024 for moderate-to-severe obstructive sleep apnoea in adults with obesity. In the UK and EU the Mounjaro brand is used for both diabetes and weight management. Both come as prefilled pens or vials made to pharmaceutical standards and supplied on prescription.

Tirzepatide is also sold online as a freeze-dried powder labelled for research use. That product is made by unidentified manufacturers and is not Mounjaro or Zepbound. Tirzepatide sits between semaglutide (GLP-1 only) and the investigational triple agonist retatrutide, which adds glucagon receptor activity.

At the GLP-1 receptor, tirzepatide has the effects familiar from other GLP-1 drugs: glucose-dependent insulin release, lower glucagon, slower stomach emptying and reduced appetite. Its GIP activity is the distinguishing feature. In Lilly’s discovery work, tirzepatide activated both receptors in cells, and in mice it reduced body weight and food intake more than a selective GLP-1 agonist did ^[1].

Exactly how GIP activity adds to the effect in people is still debated, but the clinical result is consistent: in direct comparisons, tirzepatide lowered blood glucose and body weight more than semaglutide ^{[5] [6]}.

SURMOUNT-1 was the pivotal obesity trial. It randomised 2,539 adults with a BMI of 30 or more (or 27 or more with a weight-related complication), excluding diabetes, to tirzepatide 5 mg, 10 mg or 15 mg once weekly, or placebo, for 72 weeks. Mean weight change was −15.0%, −19.5% and −20.9% on the three doses against −3.1% on placebo. At least 5% weight loss was reached by 85%, 89% and 91% of participants on the three doses, against 35% on placebo, and at the 15 mg dose 57% of participants lost 20% or more of their body weight. All prespecified cardiometabolic measures, such as waist circumference, blood pressure and lipids, improved with tirzepatide ^[2].

SURMOUNT-5 compared tirzepatide head-to-head with semaglutide in 751 adults with obesity but without type 2 diabetes. At the maximum tolerated doses, weight fell by 20.2% with tirzepatide and 13.7% with semaglutide over 72 weeks, and waist circumference fell by 18.4 cm against 13.0 cm ^[6].

In type 2 diabetes, SURPASS-1 tested tirzepatide alone against placebo in 478 people: after 40 weeks HbA1c had fallen by 1.87 to 2.07 percentage points on 5 to 15 mg, against a small rise on placebo ^[7]. SURPASS-2 compared it with semaglutide 1 mg in 1,879 people over 40 weeks. HbA1c fell by 2.01 to 2.30 percentage points with tirzepatide 5 to 15 mg against 1.86 points with semaglutide, and every tirzepatide dose was superior on this measure. Weight reductions were also greater than with semaglutide, by 1.9 kg, 3.6 kg and 5.5 kg on the three doses. Rates of nausea were similar: 17% to 22% with tirzepatide and 18% with semaglutide ^[5].

SURMOUNT-OSA comprised two 52-week trials in adults with moderate-to-severe obstructive sleep apnoea and obesity. Tirzepatide reduced the apnoea–hypopnoea index by about 20 to 24 more events per hour than placebo, in people both without and with CPAP-type therapy ^[3]. These results supported the US approval for sleep apnoea.

SUMMIT randomised 731 people with heart failure with preserved ejection fraction and obesity. Over a median follow-up of 104 weeks, cardiovascular death or worsening heart failure occurred in 9.9% on tirzepatide against 15.3% on placebo, and symptoms and physical limitations improved more with tirzepatide ^[4].

Tirzepatide’s evidence base is large: many thousands of participants across randomised, mostly placebo-controlled phase 3 trials, many published in the New England Journal of Medicine and The Lancet. As with semaglutide, this evidence describes the licensed product used as prescribed. It is not evidence about unlicensed powders, whose content and purity are unknown.

SURMOUNT-1 used 5 mg, 10 mg or 15 mg by subcutaneous injection once weekly, with a 20-week dose-escalation period before the higher doses were reached ^[2]. SURPASS-2 used the same three maintenance doses ^[5]. SURMOUNT-OSA and SURMOUNT-5 used the maximum tolerated dose, 10 mg or 15 mg weekly ^{[3] [6]}, and SUMMIT used doses up to 15 mg weekly ^[4]. Lilly’s phase 1 work explored single doses of 0.25 to 8 mg and multiple doses up to 15 mg ^[1].

These are the doses used in trials of the licensed medicine, reported for information, not as a recommendation. They cannot be applied to research-grade vials whose true peptide content has not been verified. Our peptide calculator covers the arithmetic of reconstitution only.

The most common side effects were gastrointestinal (nausea, diarrhoea, vomiting and constipation), mostly mild to moderate and concentrated in the dose-escalation period. In SURMOUNT-1, adverse events led to stopping treatment in 4.3%, 7.1% and 6.2% of participants on 5, 10 and 15 mg, against 2.6% on placebo ^[2]. In SUMMIT, 6.3% stopped tirzepatide because of adverse events, mainly gastrointestinal, against 1.4% on placebo ^[4].

In SURPASS-2, serious adverse events were reported in 5% to 7% of tirzepatide users and 3% of semaglutide users, and low blood glucose (below 54 mg/dL) was uncommon in both groups ^[5]. Prescribing information for tirzepatide, like other GLP-1-based medicines, also lists risks such as pancreatitis and gallbladder disease, and the US label carries a boxed warning about thyroid C-cell tumours seen in rodents.

For unlicensed products, the risks from uncertain dose, impurities and contamination are added to the drug’s known side effects. Testing of research-grade GLP-1 peptides bought online has found large deviations from the labelled content (see our semaglutide and retatrutide guides for published examples).

Tirzepatide is a licensed prescription-only medicine. In the US, Mounjaro is approved for type 2 diabetes (May 2022) and Zepbound for chronic weight management (November 2023) ^[8] and for obstructive sleep apnoea in adults with obesity (December 2024) ^[9]. It is also licensed in the UK, the EU and other markets, where Mounjaro is used for both diabetes and weight management.

Unlicensed tirzepatide sold as a “research chemical” is not covered by any of these approvals. How the law treats its sale and possession differs between countries; see our legal status overview and the UK page.

Licensed tirzepatide pens and vials are stored in a refrigerator and must not be frozen; the product leaflet states how long they can be kept unrefrigerated. Research-grade tirzepatide is usually supplied freeze-dried and is generally kept cold, dry and dark, and refrigerated after reconstitution. See how to store peptides.

PepFinder does not sell tirzepatide. To compare research suppliers, see compare prices, read how to read a COA, and check which vendors provide third-party tested batches. A certificate from an independent laboratory confirming identity and quantity matters more than price.