Tirzepatide works by switching on two gut-hormone receptors at once: the GIP receptor and the GLP-1 receptor. Those signals lower appetite, slow how fast food leaves the stomach, and help the pancreas release insulin when blood sugar is high. The result in trials was less food intake and large weight loss over 72 weeks. Start with our tirzepatide guide if you are new to the drug.
Key takeaways
- The label describes tirzepatide as a GIP receptor and GLP-1 receptor agonist: it mimics two natural hormones your gut releases after you eat.
- It is a synthetic peptide of 39 amino acids with an attached fatty acid chain, made in a lab by Eli Lilly, not taken from an animal or plant.
- Its half-life is about 5 days, which is why it is injected once a week and reaches steady state after about 4 weeks.
- In SURMOUNT-1, the 15 mg dose produced a mean weight change of -20.9% at 72 weeks versus -3.1% for placebo; in SURPASS-1, A1c fell by up to 2.07 points in type 2 diabetes at 40 weeks.
- Compounded tirzepatide contains the same active molecule but is not FDA approved and has not been tested in these trials.
Where does tirzepatide come from?
Tirzepatide is not a natural hormone. It is a designed molecule that borrows its shape from one, GIP (glucose-dependent insulinotropic polypeptide), and is then modified so it can also bind the GLP-1 (glucagon-like peptide-1) receptor. The label describes it as a 39-amino-acid peptide with a C20 fatty diacid chain attached. That fatty chain binds to albumin, a protein in your blood, which slows the drug's clearance and gives it the 5-day half-life that makes once-weekly dosing possible.
Eli Lilly developed it and makes the branded versions, Mounjaro (approved for type 2 diabetes in May 2022) and Zepbound (approved for chronic weight management in November 2023 and for obstructive sleep apnea in adults with obesity in December 2024). Compounding pharmacies buy tirzepatide as a raw ingredient and prepare it into vials for individual patients. Our tirzepatide peptide page covers the molecule in more detail.
Tirzepatide mechanism of action: two receptors, one drug
After a meal, your gut releases GIP and GLP-1. Both are incretin hormones: they tell the pancreas to release insulin in response to food, and they signal to the brain that you have eaten. Natural incretins are broken down within minutes. Tirzepatide copies their signal and stays active for days.
Check your GLP-1 eligibility
Use our free BMI Calculator to see if you may qualify for provider-reviewed GLP-1 therapy.
Try the BMI Calculator →GLP-1 receptor: appetite and stomach emptying
Activating GLP-1 receptors in the brain reduces appetite and food intake. In the gut it slows gastric emptying, so food leaves the stomach more slowly and you feel full longer. The label notes this delay in gastric emptying as the reason tirzepatide can change how oral medicines are absorbed, which is why the label tells people on oral contraceptives to add a non-oral or barrier method for 4 weeks after starting and after each dose increase.
GIP receptor: the second signal
GIP is the other incretin. The GIP receptor is found in the pancreas and in fat tissue. Adding GIP activity to GLP-1 activity is what sets tirzepatide apart from single-receptor drugs like semaglutide. The label does not assign a percentage of the effect to each receptor, so this page does not either. What the trials show is the combined result: in SURMOUNT-5, tirzepatide produced -20.2% mean weight change versus -13.7% for semaglutide at 72 weeks in adults with obesity without diabetes.
Insulin and glucagon
Both receptors increase insulin release from the pancreas when glucose is high, and GLP-1 activity lowers glucagon, the hormone that raises blood sugar. Because the insulin effect is glucose-dependent, tirzepatide on its own carries a low risk of low blood sugar. The label warns that the risk rises when it is combined with insulin or sulfonylureas, and that those drugs may need a lower dose.
What the mechanism produces: SURMOUNT-1 by dose
The clearest evidence for the mechanism is the dose-response pattern. More receptor activation, more weight loss, more gastrointestinal side effects.
| SURMOUNT-1, 72 weeks | Placebo | 5 mg | 10 mg | 15 mg |
|---|---|---|---|---|
| Mean weight change | -3.1% | -15.0% | -19.5% | -20.9% |
| Lost at least 5% of body weight | 35% | 85% | 89% | 91% |
| Lost at least 20% of body weight | 3% | Not in summary | 50% | 57% |
| Nausea | 9.5% | 24.6% | 33.3% | 31.0% |
| Diarrhea | 7.3% | 18.7% | 21.2% | 23.0% |
| Stopped for adverse events | 2.6% | 4.3% | 7.1% | 6.2% |
Source: SURMOUNT-1, Jastreboff et al., NEJM 2022 (PubMed 35658024); 2,539 adults with obesity or overweight without diabetes.
The same mechanism in type 2 diabetes: SURPASS-1
Because the insulin and glucagon effects lower blood sugar, tirzepatide was first approved for type 2 diabetes. SURPASS-1 tested it alone, without other diabetes drugs, for 40 weeks.
| SURPASS-1, 40 weeks | Placebo | 5 mg | 10 mg | 15 mg |
|---|---|---|---|---|
| Change in A1c (percentage points) | +0.04 | -1.87 | -1.89 | -2.07 |
Source: SURPASS-1, Rosenstock et al., Lancet 2021 (PubMed 34186022).
In SURPASS-2, tirzepatide was compared head to head with semaglutide 1 mg in type 2 diabetes: A1c fell 2.01 to 2.30 points on tirzepatide versus 1.86 on semaglutide, and weight fell 7.6 to 11.2 kg versus 5.7 kg. See our tirzepatide and insulin resistance page for the diabetes trials in full.
Tirzepatide pharmacology in plain terms
| Property | What the label says | Why it matters |
|---|---|---|
| Route | Subcutaneous injection, abdomen, thigh or upper arm | A peptide cannot survive digestion, so it is injected |
| Half-life | About 5 days | Once-weekly dosing; steady state after about 4 weeks |
| Timing | Any time of day, with or without meals | The effect does not depend on meal timing |
| Dose range | 2.5 mg start, then 5 to 15 mg in 2.5 mg steps | Slow escalation limits gastrointestinal side effects |
| Missed dose | Take within 4 days, otherwise skip | Keeps blood levels from stacking |
Source: Zepbound and Mounjaro prescribing information (DailyMed), revised 08/2026.
What this means for compounded tirzepatide
The mechanism belongs to the molecule, so a correctly made compounded tirzepatide vial acts on the same two receptors. What a compounded product does not have is the trial evidence and FDA review behind the branded product. Compounded tirzepatide is not FDA approved and is not interchangeable with Zepbound or Mounjaro. At FormBlends it is prescribed by independent clinicians of Recess MD LLC, prepared by state-licensed 503A compounding pharmacies, and priced at $149 for the first month and $239 a month after that at any dose. See pricing or the assessment.
Frequently Asked Questions
How does tirzepatide work for weight loss?
It activates GIP and GLP-1 receptors, which reduces appetite and food intake and slows stomach emptying. Eating less over many months is what drives the weight loss seen in SURMOUNT-1, where the 15 mg dose produced a -20.9% mean weight change at 72 weeks.
What is the tirzepatide MOA in one sentence?
Tirzepatide is a once-weekly GIP and GLP-1 receptor agonist: it mimics two incretin hormones to increase glucose-dependent insulin release, lower glucagon, slow gastric emptying and reduce appetite.
What does GIP do in tirzepatide?
GIP is the second incretin hormone tirzepatide copies. Its receptor sits in the pancreas and fat tissue and adds to the insulin and appetite effects of GLP-1. The label does not split the effect between the two receptors, so no percentage can be given.
Where does tirzepatide come from?
It is made in a laboratory by Eli Lilly as a 39-amino-acid synthetic peptide with an attached fatty acid chain. It is sold as Mounjaro and Zepbound, and compounding pharmacies also prepare it from the raw ingredient for individual prescriptions.
What is the mechanism of action of Zepbound?
Zepbound is tirzepatide, so its mechanism is the same dual GIP and GLP-1 receptor agonism described on this page. It is approved for chronic weight management and for moderate to severe obstructive sleep apnea in adults with obesity.
Sources
- Zepbound prescribing information, DailyMed, revised 08/2026.
- Mounjaro prescribing information, DailyMed, revised 08/2026.
- Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1), Jastreboff et al., New England Journal of Medicine, 2022.
- Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5), Aronne et al., New England Journal of Medicine, 2025.
- Efficacy and safety of a novel dual GIP and GLP-1 receptor agonist tirzepatide in patients with type 2 diabetes (SURPASS-1), Rosenstock et al., Lancet, 2021.
- Tirzepatide versus Semaglutide Once Weekly in Patients with Type 2 Diabetes (SURPASS-2), Frias et al., New England Journal of Medicine, 2021.
- FDA Approves New Medication for Chronic Weight Management, U.S. Food and Drug Administration, November 8, 2023.
Next steps
Read how long tirzepatide takes to work for the timeline, or compare the two-receptor drug with a one-receptor drug in semaglutide vs tirzepatide. Browse the tirzepatide hub for every other tirzepatide question. Talk to a licensed clinician before starting or changing any medication. Compounded tirzepatide is not FDA approved and is not interchangeable with Zepbound or Mounjaro; a licensed provider decides whether it is appropriate for you.
See your options in about 2 minutes
Take the free quiz and see what fits you. Quick, private, and no commitment to continue.
See my options →