Tirzepatide
A weekly GIP/GLP-1 injection, sold by Lilly as Mounjaro and Zepbound. What it does, what the trials found, every dose, and the three ways people get it.
- Tirzepatide activates two gut-hormone receptors, GIP and GLP-1, with one molecule.
- In SURMOUNT-1, people lost 15.0% to 20.9% of their weight over 72 weeks depending on dose, against 3.1% on placebo.
- It starts at 2.5 mg weekly and rises in 2.5 mg steps every four weeks or more, to 15 mg.
- Most side effects are digestive and peak after dose increases. Stopping usually brings the weight back.
What the evidence shows
A: several large randomised trials. B: one or two randomised trials. C: smaller or indirect evidence.
What is tirzepatide?
Tirzepatide is a weekly injected medicine made by Eli Lilly. It is sold as Mounjaro for type 2 diabetes and as Zepbound for weight management and obstructive sleep apnoea.
Who it is approved for
Mounjaro: adults with type 2 diabetes. Zepbound: adults with obesity, or overweight with a weight-related condition, and adults with moderate to severe sleep apnoea and obesity.
Ways to get tirzepatide
How tirzepatide works
The two pathways
GLP-1 signals fullness, slows stomach emptying and raises insulin when blood sugar is high. GIP also raises insulin after meals and appears to add to the effect on appetite. Tirzepatide activates both.
The long half-life
A fatty-acid chain keeps it in the blood for about five days, which is why it is injected once a week.
Research findings
SURMOUNT-1: weight loss
2,539 adults with obesity and no diabetes. At 72 weeks: 15.0% at 5 mg, 19.5% at 10 mg, 20.9% at 15 mg, against 3.1% on placebo1.
SURPASS-2: against semaglutide
In type 2 diabetes, all three doses lowered HbA1c and weight more than semaglutide 1 mg over 40 weeks2.
SURMOUNT-4: stopping
People switched to placebo after 36 weeks regained much of the weight, while those who continued kept losing3.
Dosage & administration
| Step | Weekly dose | When | Page |
|---|---|---|---|
| Start | 2.5 mg | weeks 1–4, not a maintenance dose | 2.5 mg → |
| Step 2 | 5 mg | after at least 4 weeks | 5 mg → |
| Step 3 | 7.5 mg | after at least 4 more weeks | 7.5 mg → |
| Step 4 | 10 mg | after at least 4 more weeks | 10 mg → |
| Step 5 | 12.5 mg | after at least 4 more weeks | 12.5 mg → |
| Maximum | 15 mg | after at least 4 more weeks | 15 mg → |
On the Zepbound label the maintenance doses are 5, 10 or 15 mg for weight management and 10 or 15 mg for sleep apnoea4.
Work out your tirzepatide draw in units
Safety & side effects
Common side effects
Nausea, diarrhoea, vomiting and constipation, most often in the weeks after each dose increase.
Other label warnings
Severe stomach side effects, kidney injury from dehydration, gallbladder disease, pancreatitis, allergic reactions, low blood sugar with insulin or sulfonylureas, diabetic eye disease, and aspiration during anaesthesia4.
Frequently asked questions
What is the difference between Mounjaro, Zepbound and research tirzepatide?
Mounjaro and Zepbound are the same Lilly molecule approved for different conditions. Research tirzepatide is sold as a laboratory chemical, is not approved for use in people and comes with no prescriber.
How much weight do people lose on tirzepatide?
15.0% to 20.9% on average over 72 weeks in SURMOUNT-1, depending on dose, against 3.1% on placebo.
How does tirzepatide compare to semaglutide?
In SURPASS-2, all three doses lowered HbA1c and weight more than semaglutide 1 mg in type 2 diabetes.
What happens if I stop tirzepatide?
In SURMOUNT-4, people switched to placebo regained much of the weight they had lost.
Scientific references
- Jastreboff AM et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). N Engl J Med 2022.
- Frías JP et al. Tirzepatide versus semaglutide once weekly in patients with type 2 diabetes (SURPASS-2). N Engl J Med 2021.
- Aronne LJ et al. Continued treatment with tirzepatide for maintenance of weight reduction (SURMOUNT-4). JAMA 2024.
- Eli Lilly and Company. ZEPBOUND (tirzepatide) prescribing information. DailyMed, 2026.