Tirzepatide is a once-weekly injection from Eli Lilly that activates two hormone receptors, GIP and GLP-1.[1] It is FDA-approved as Mounjaro for type 2 diabetes and as Zepbound for chronic weight management and obstructive sleep apnea.[2][1] In its main obesity trial, people on the highest dose lost an average of 20.9% of their body weight over 72 weeks.[6]
This profile covers how tirzepatide works, its FDA approvals, the key trial results and its side effects. See also our profiles of retatrutide and semaglutide.
What is tirzepatide?
Tirzepatide is a single synthetic peptide that acts on both the GIP and GLP-1 receptors, which is why it is called a “dual agonist”.[7] It is sold under two brand names with different approved uses:
- Mounjaro: approved in May 2022 to improve blood sugar control in adults with type 2 diabetes.[5]
- Zepbound: approved in November 2023 for chronic weight management, and in December 2024 for moderate to severe obstructive sleep apnea in adults with obesity.[3][4]
Both are used together with a reduced-calorie diet and more physical activity.[1]
How tirzepatide works
GLP-1 and GIP are incretin hormones, which the gut releases after eating. Tirzepatide activates the receptors for both of them.[1] Semaglutide activates only the GLP-1 receptor, and the investigational drug retatrutide adds a third, the glucagon receptor.
Tirzepatide’s half-life is about 5 to 6 days, which allows once-weekly injections.[1]
Clinical trial results
Weight loss: SURMOUNT-1
SURMOUNT-1 enrolled adults with obesity, or overweight plus a weight-related condition, who did not have diabetes. Average starting weight was 104.8 kg.[6]
| Dose | Average weight change at 72 weeks | Lost 5% or more |
|---|---|---|
| 5 mg | −15.0% | 85% |
| 10 mg | −19.5% | 89% |
| 15 mg | −20.9% | 91% |
| Placebo | −3.1% | 35% |
Half or more of people on 10 mg and 15 mg lost at least 20% of their body weight, compared with 3% on placebo.[6]
Three-year results and diabetes prevention
Participants with prediabetes continued in SURMOUNT-1 for 176 weeks. Weight loss held at 12.3% to 19.7% depending on dose, versus 1.3% on placebo. Only 1.3% of people on tirzepatide developed type 2 diabetes, compared with 13.3% on placebo.[9]
Type 2 diabetes: SURPASS-1
In 478 adults with type 2 diabetes, HbA1c fell by 1.87 to 2.07 percentage points after 40 weeks, versus a 0.04-point rise on placebo, without an increased risk of low blood sugar.[7]
Obstructive sleep apnea: SURMOUNT-OSA
In two trials of adults with moderate to severe sleep apnea and obesity, tirzepatide reduced breathing interruptions during sleep (the apnea-hypopnea index) by 25.3 and 29.3 events per hour at 52 weeks, versus 5.3 and 5.5 on placebo.[8]
Side effects
The most common side effects in trials were gastrointestinal, mostly mild to moderate, and occurred mainly while the dose was being increased.[6]
- In SURMOUNT-1, 4.3% to 7.1% of people on tirzepatide stopped treatment because of side effects, versus 2.6% on placebo.[6]
- The three-year results found no new safety signals.[9]
Boxed warning: the label warns that tirzepatide caused thyroid C-cell tumors in rats. It is unknown whether it causes these tumors, including medullary thyroid carcinoma, in humans.[1] Read the full prescribing information for all warnings.
Doses studied
The main trials tested maintenance doses of 5 mg, 10 mg and 15 mg once a week.[6][7] Your prescriber decides the dose and how quickly to increase it, following the prescribing information.
Tirzepatide vs semaglutide and retatrutide
Tirzepatide’s weight loss in SURMOUNT-1 was larger than semaglutide’s in its main obesity trial, but comparing separate trials is unreliable because the populations and methods differ. See our semaglutide and retatrutide profiles for each drug’s own evidence.