Plain-English explainer
What is tirzepatide?
Tirzepatide is a once-weekly injection developed by Eli Lilly. In the UK it is sold as Mounjaro. Here is what it is, how it works and who it is for, in plain English.
One paragraph
Tirzepatide is a once-weekly injection developed by Eli Lilly to lower blood sugar in type 2 diabetes and to support weight loss in people with obesity. It is sold in the UK under the brand name Mounjaro and is licensed by the MHRA for both uses. The active ingredient is a single 39-amino-acid peptide that simultaneously activates two natural appetite-control receptors in the body — GLP-1 and GIP.
How it works (briefly)
The body has built-in "incretin" hormones that are released when you eat. They signal the pancreas to release insulin, slow the rate at which the stomach empties, and tell the brain you are full. Tirzepatide mimics two of these hormones at once. The combined effect is a reduction in appetite, smaller meals, slower digestion, and (over weeks) a meaningful loss of body fat.
What the trials showed
The major studies are SURMOUNT-1 (people with obesity without diabetes) and SURPASS-2 (people with type 2 diabetes versus semaglutide). In SURMOUNT-1, the 15 mg weekly dose produced a mean body-weight reduction of ~22.5% at 72 weeks — larger than any other approved weight medicine. SURPASS-2 showed tirzepatide beating semaglutide on both HbA1c reduction and weight loss.
How do you take it?
As a subcutaneous injection, once a week, with a prefilled pen called KwikPen. The dose starts low (2.5 mg) and steps up every four weeks to a maximum of 15 mg, which most people reach by week 20. The full titration schedule is on the tirzepatide dosing page.
Side effects
The most common are gastrointestinal: nausea (about one in four people), diarrhoea, constipation and reduced appetite (which is partly the intended effect). Most settle as you adjust to each dose step. Rare but serious side effects include acute pancreatitis and gallstones, both more likely with rapid weight loss. The full safety profile is on the tirzepatide profile.
Who is it for?
In the UK, MHRA-licensed indications cover (1) adults with type 2 diabetes and (2) adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one weight-related condition. Outside those licensed pathways it should not be used. Always consult a registered UK clinician.
Read more: Full tirzepatide profile · Mounjaro brand page · Semaglutide vs tirzepatide.
FAQs
- Is tirzepatide the same as Mounjaro?
- Yes — Mounjaro is the UK brand name for the molecule tirzepatide. Both names refer to exactly the same drug. Eli Lilly sells it as Mounjaro in the UK and EU; in the US, the weight-management version uses the brand Zepbound, while Mounjaro stays for the type-2-diabetes indication.
- Is tirzepatide a GLP-1 like Ozempic?
- Tirzepatide is similar in family but more powerful. Ozempic (semaglutide) targets only the GLP-1 receptor. Tirzepatide targets both GLP-1 and a second incretin receptor called GIP. The combination produces substantially larger weight reductions in clinical trials.
- How much weight does tirzepatide cause people to lose?
- In the SURMOUNT-1 trial (NEJM 2022) participants without diabetes lost a mean of approximately 22.5% of their body weight at the 15 mg weekly dose over 72 weeks. That is the largest effect of any approved weight-management medicine to date.
- Is tirzepatide safe?
- Tirzepatide has a well-characterised safety profile from large Phase 3 trials. The most common side effects are gastrointestinal — nausea, diarrhoea and constipation — which usually settle as the dose escalates slowly over 24 weeks. Rare but serious risks include pancreatitis and gallstones with rapid weight loss. The cardiovascular outcomes data (SURMOUNT-MMO) is still being collected.