GLP-1 & incretins

Tirzepatide (Mounjaro)

Reviewed 22 October 2026 · How we research

Also known as: Mounjaro, Zepbound, Tirz, TZ, LY3298176

What is Tirzepatide?

Tirzepatide is the active ingredient in Mounjaro. Yes — tirzepatide and Mounjaro are the same drug; Mounjaro is Eli Lilly's brand name (and Zepbound is the US brand for weight management). It is a once-weekly injection that activates both the GIP and GLP-1 receptors, which is why it is called a “dual agonist” or “twincretin”.

Unlike retatrutide, tirzepatide is a licensed medicine in the UK. It is prescribed for type 2 diabetes and for weight management in adults meeting BMI criteria, through the NHS and private online pharmacies.

How Tirzepatide works

GLP-1 receptor activation slows stomach emptying and signals fullness to the brain. GIP receptor activation was once expected to work against weight loss, but tirzepatide showed the combination produces more appetite suppression and better insulin sensitivity than GLP-1 activation alone. The peptide is built on the GIP sequence and modified with a C20 fatty-diacid chain that binds albumin, giving the five-day half-life.

What the research shows

• SURMOUNT-1 (2,539 adults with obesity, 72 weeks): mean weight change −15.0% (5 mg), −19.5% (10 mg) and −20.9% (15 mg) versus −3.1% on placebo.
• SURPASS-2 (type 2 diabetes): all tirzepatide doses beat semaglutide 1 mg on HbA1c and weight reduction over 40 weeks.
• SURMOUNT-5 head-to-head in obesity: tirzepatide (max tolerated 10–15 mg) produced −20.2% weight change versus −13.7% for semaglutide (1.7–2.4 mg) at 72 weeks.
• SURMOUNT-OSA: tirzepatide reduced sleep-apnoea severity, leading to a US approval for obstructive sleep apnoea in people with obesity.

Compare: Tirzepatide vs Semaglutide · Retatrutide vs Tirzepatide

Tirzepatide side effects

• Gastrointestinal: nausea, diarrhoea, constipation, vomiting and indigestion — most common in the first weeks after each dose increase.
• Reduced appetite, burping, abdominal pain, fatigue.
• Less common but serious: gallbladder problems, pancreatitis, severe dehydration from vomiting/diarrhoea.
• The MHRA has warned that Mounjaro may reduce the effectiveness of oral contraceptives in people who are overweight, particularly after starting and after each dose increase.

Carries the class warning about thyroid C-cell tumours seen in rodents; it is not used in people with a personal or family history of medullary thyroid cancer.

UK legal status

Tirzepatide is licensed in the UK as Mounjaro. NICE recommended it for weight management (TA1026, December 2024), with a phased NHS rollout through GP services that began in June 2025 for people with the highest clinical need. Most UK users currently obtain it on private prescription from online pharmacies, which must carry out a clinical consultation.

Lilly raised the UK list price of Mounjaro from September 2025, which pushed private prices up and drove interest in “tirzepatide UK” price comparisons. Tirzepatide sold as a lyophilised research chemical is not Mounjaro and is not licensed for human use.

More detail, including current UK research-vial prices: Tirzepatide UK guide. For the doses used in published studies and the reconstitution maths: Tirzepatide dosing in the literature.

References

1. Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387:205–216.
2. Frías JP, et al. Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes. N Engl J Med. 2021;385:503–515.
3. Aronne LJ, et al. Tirzepatide as compared with semaglutide for the treatment of obesity (SURMOUNT-5). N Engl J Med. 2025.
4. Malhotra A, et al. Tirzepatide for the treatment of obstructive sleep apnea and obesity. N Engl J Med. 2024;391:1193–1205.