The Short Answer
Tirzepatide is a once-weekly injectable peptide that activates two gut-hormone receptors at once: GIP and GLP-1. Eli Lilly sells it as Mounjaro for type 2 diabetes (FDA-approved May 2022) and Zepbound for chronic weight management (November 2023), with obstructive sleep apnea added to the Zepbound label later.
In its pivotal obesity trial, people taking the highest dose lost an average of 20.9% of their body weight over 72 weeks, the largest result for any approved weight-loss drug at the time. The trade-off is the GLP-1 class side-effect profile: mostly gastrointestinal, mostly during dose increases.
This guide explains what tirzepatide is and what the trials reported. It does not recommend doses; the label and a prescriber do that.
Tirzepatide at a Glance
| Type | Synthetic 39-amino-acid peptide; GIP and GLP-1 receptor agonist ("dual agonist") |
| Brands | Mounjaro (type 2 diabetes), Zepbound (weight management; obstructive sleep apnea) |
| Maker | Eli Lilly |
| FDA approvals | Mounjaro: May 13, 2022. Zepbound: November 8, 2023 |
| How taken | Once-weekly subcutaneous injection (pen or vial) |
| Pivotal weight trial | SURMOUNT-1: 15.0%, 19.5% and 20.9% mean weight loss at 72 weeks on 5, 10 and 15 mg vs 3.1% on placebo |
| Status | Approved, prescription-only |
How Tirzepatide Works
GLP-1 and GIP are incretin hormones your gut releases after meals. Both increase insulin release when blood sugar is high. GLP-1 also slows stomach emptying and reduces appetite through the brain. Tirzepatide is engineered to bind both receptors and to last about a week in the body.
Why two receptors? Semaglutide activates GLP-1 alone and produced about 15% weight loss in its pivotal trial. Adding GIP activity appears to increase both weight loss and glucose control, although exactly how GIP contributes is still debated. Retatrutide goes one step further with a third receptor, glucagon. The mechanism comparison walks through all three.
What the Trials Showed
SURMOUNT-1 (obesity without diabetes, NEJM 2022): 2,539 adults with obesity, or overweight plus a weight-related condition, were randomized to once-weekly tirzepatide 5, 10 or 15 mg or placebo for 72 weeks, including a 20-week dose-escalation period. Mean weight change was -15.0%, -19.5% and -20.9% versus -3.1% on placebo. Half or more of participants on the two higher doses lost 20% or more of their body weight. All prespecified cardiometabolic measures improved. Discontinuation for adverse events was 4.3% to 7.1% on tirzepatide versus 2.6% on placebo.
Type 2 diabetes: the SURPASS program supported the Mounjaro approval, showing reductions in A1C alongside weight loss.
Obstructive sleep apnea: Zepbound's label now includes moderate to severe OSA in adults with obesity, based on the SURMOUNT-OSA trials.
The label describes how tirzepatide is started and increased; those steps are set by the prescriber and are not repeated here as instructions.
Side Effects Reported in Trials
- Gastrointestinal: nausea, diarrhea, vomiting and constipation were the most common adverse events in SURMOUNT-1, mostly mild to moderate and concentrated during dose escalation.
- Class warnings: like other GLP-1 receptor agonists, tirzepatide carries a boxed warning about thyroid C-cell tumors seen in rodents and is contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2. The label also discusses pancreatitis, gallbladder problems, low blood sugar when combined with insulin or sulfonylureas, and delayed stomach emptying.
- Lean mass: weight loss includes some muscle. See GLP-1 lean mass meta-analysis.
A prescriber reviews these against your history. If you're having side effects on tirzepatide, that's a conversation with the clinic, not a reason to adjust on your own.
Tirzepatide vs Semaglutide vs Retatrutide
| Semaglutide (Wegovy) | Tirzepatide (Zepbound) | Retatrutide | |
|---|---|---|---|
| Receptors | GLP-1 | GIP + GLP-1 | GIP + GLP-1 + glucagon |
| FDA status | Approved (2021; oral tablet 2025) | Approved (2023) | Investigational; Lilly plans to file in Q1 2027 |
| Pivotal trial weight loss | 14.9% at 68 weeks (STEP 1) | 20.9% at 72 weeks, highest dose (SURMOUNT-1) | 24.2% at 48 weeks, 12 mg (Phase 2) |
| Legal supply | Prescription | Prescription | None |
Head-to-head: retatrutide vs tirzepatide clinical data. Considering a change later: switching from tirzepatide to retatrutide explains why that isn't possible through legal channels today.
Compounded Tirzepatide
FDA's guidance on unapproved GLP-1 products warns that compounded tirzepatide is not FDA-reviewed, that dosing errors have been reported with vial-and-syringe products, and that marketing a compounded drug as "the same" as the approved one is a red flag. FDA has also reported adverse events, some requiring hospitalization, that may be related to dosing errors with compounded injectable GLP-1 products. Read FDA's concerns with unapproved GLP-1 drugs before using any non-pharmacy source.
Licensed GLP-1 telehealth (partner)
Explore tirzepatide or semaglutide treatment options online with a licensed clinician if appropriate for you.
