What is tirzepatide? Mounjaro and Zepbound, and what GIP adds, in plain English

The short version

Tirzepatide is a weekly injection that switches on two gut-hormone receptors, GIP and GLP-1. It is sold as Mounjaro and Zepbound, and its half-life is about 5 days.^1,2
In SURMOUNT-1, adults with obesity and no diabetes lost an average of 15.0% to 20.9% of body weight over 72 weeks, depending on dose, vs 3.1% on placebo.⁴
In a head-to-head trial, SURMOUNT-5, average weight loss was 20.2% on tirzepatide vs 13.7% on semaglutide.⁵

What is tirzepatide?

Tirzepatide is a once-weekly injected medicine that copies two hormones your gut releases after you eat: GIP and GLP-1. The labels call it a GIP receptor and GLP-1 receptor agonist. A receptor is a docking spot on a cell, and an agonist is a drug that switches on the same receptor the body's own hormone does. By switching on both, tirzepatide helps the body release insulin when blood sugar is high and turns down appetite.^1,2 Tirzepatide is the generic name. Mounjaro and Zepbound are Eli Lilly's brand names for it.

An everyday picture: if semaglutide is one key that fits one lock (the GLP-1 receptor), tirzepatide is a single key cut to fit two locks. The analogy stops working because the two locks are not equal partners, and scientists are still working out how much the GIP part adds in people.

Illustration of a vial and a pen side by side.

What is GIP, and how is it different from GLP-1?

GIP stands for glucose-dependent insulinotropic polypeptide. "Insulinotropic" means it prompts the pancreas to release insulin. GIP and GLP-1 are the body's two main incretin hormones: gut hormones released when you eat that help the pancreas release insulin, but only when blood sugar is raised.³ In the brain, both are thought to play a part in appetite.³

The Zepbound label puts it this way: GLP-1 regulates appetite and calorie intake, and studies in animals and cells "suggest the addition of GIP may further contribute" to regulating food intake. Receptors for both hormones are found in brain areas that control appetite.¹ For a primer on the GLP-1 side, see what is GLP-1?

What is tirzepatide made of?

Like semaglutide, tirzepatide is a peptide, a short chain of amino acids. The label says it is based on the GIP sequence, with a few changes:¹

Two unusual amino acids (aminoisobutyric acid) at positions 2 and 13.
A 20-carbon fatty acid chain attached through a linker. It lets tirzepatide stick to albumin, a common protein in the blood, which makes it last much longer.

The single-dose pens and vials contain tirzepatide, salt (sodium chloride), sodium phosphate and water. The 4-dose KwikPen and multi-dose vial also contain preservatives and other ingredients: benzyl alcohol, glycerin and phenol.¹

Mounjaro vs Zepbound: what is each approved for?

Sources: Zepbound and Mounjaro US labels, section 1.^1,2 Zepbound's uses come with a reduced-calorie diet and more physical activity.
Brand	US approved uses (label section 1)
Mounjaro	Blood sugar control in type 2 diabetes, in adults and children 10 and older; lowering the risk of heart attack, stroke and heart-related death in adults with type 2 diabetes at high risk of these events
Zepbound	Long-term weight management in adults with obesity, or overweight plus at least one weight-related condition; moderate to severe obstructive sleep apnea in adults with obesity

Obstructive sleep apnea is when the airway repeatedly collapses during sleep, cutting off breathing. The Zepbound label says this approval is based on two placebo-controlled trials in 467 adults with moderate to severe sleep apnea and obesity (label section 14), published as SURMOUNT-OSA.^1,6 See GLP-1s and sleep apnea. The Zepbound label also says using it with other tirzepatide products or with any GLP-1 medicine is not recommended.¹

Pens vs vials

Both brands come in the same six strengths, from 2.5 mg to 15 mg (label section 3). The label lists four forms:^1,2

Single-dose pen: a prefilled pen, used once.
Single-dose vial: a small bottle drawn up with a syringe, used once.
Multi-dose vial: 2.4 mL holding 4 doses.
KwikPen: a pen holding 4 doses, used by one person.

Vials mean drawing up the dose yourself, so ask your pharmacist or care team to show you how. For storage, see how to store Ozempic and Zepbound, and for prices, Zepbound and Wegovy cost.

How long does tirzepatide last in the body?

Half-life is the time it takes for half of a drug to leave the blood. In the labels' pharmacokinetics section (12.3), tirzepatide's half-life is about 5 days (Mounjaro) and about 5 to 6 days in people with overweight or obesity (Zepbound).^1,2 The Mounjaro label says this is what allows once-weekly dosing. Semaglutide's half-life is a little longer, about a week. More in our GLP-1 half-life explainer.

What did the big trials find?

−20.9%average weight change at 72 weeks on tirzepatide 15 mg vs −3.1% on placebo, SURMOUNT-1, 2,539 adults without diabetes

−20.2% vs −13.7%tirzepatide vs semaglutide at 72 weeks, SURMOUNT-5, 751 adults with obesity and no type 2 diabetes

SURMOUNT-1. Adults with a BMI of 30 or more, or 27 or more with a weight-related condition, and no diabetes got tirzepatide 5, 10 or 15 mg or placebo for 72 weeks. Average weight fell 15.0%, 19.5% and 20.9% on the three doses, and 3.1% on placebo. On 15 mg, 57% lost at least a fifth of their body weight, vs 3% on placebo.⁴

Tirzepatide 15 mg−20.9%

Tirzepatide 10 mg−19.5%

Tirzepatide 5 mg−15.0%

Placebo−3.1%

SURMOUNT-5 (tirzepatide vs semaglutide). In this open-label trial, meaning everyone knew which drug they got, 751 adults with obesity and no type 2 diabetes took the highest dose they tolerated of tirzepatide or semaglutide for 72 weeks. Weight fell 20.2% on tirzepatide and 13.7% on semaglutide. Waist size shrank 18.4 cm vs 13.0 cm.⁵ Full details in semaglutide vs tirzepatide, and see what is semaglutide? for the other drug.

Side effects and the boxed warning

Stomach side effects are the most common. The Zepbound label lists those reported in at least 5% of patients: nausea, diarrhea, vomiting, constipation, belly pain, indigestion, injection site reactions, tiredness, allergic-type reactions, burping, hair loss and reflux.¹ In SURMOUNT-1, most stomach side effects were mild to moderate and happened mainly while the dose was being stepped up. Side effects led 4.3% to 7.1% of people on tirzepatide to stop, depending on dose, vs 2.6% on placebo.⁴

Both tirzepatide labels carry a boxed warning, the FDA's strongest, about thyroid C-cell tumors. Tirzepatide caused these tumors in rats, and it is unknown whether it does in humans. The labels say not to use it if you or a family member has had medullary thyroid cancer or if you have MEN 2.^1,2 A lump in the neck, trouble swallowing or ongoing hoarseness should be checked by a doctor, and severe belly pain that will not go away needs urgent care.

More in nausea on GLP-1s and gallbladder and pancreatitis risks.

Key terms in this article

GIP: glucose-dependent insulinotropic polypeptide, a gut hormone that prompts insulin release after eating.
GLP-1: glucagon-like peptide-1, a gut hormone that helps control blood sugar and appetite.
Incretin: a gut hormone, like GIP or GLP-1, that boosts insulin release after a meal.
Receptor agonist: a drug that switches on the same receptor (cell docking spot) as a natural hormone.
Half-life: the time it takes for half of a drug to leave the blood.
Open-label trial: a trial where participants and doctors know which treatment each person gets.
Boxed warning: the FDA's most serious type of label warning.

Common questions

What is tirzepatide used for?

In the US, tirzepatide is approved as Mounjaro for blood sugar control in type 2 diabetes (adults and children 10 and older) and to lower heart risk in adults with type 2 diabetes at high risk. As Zepbound, it is approved for long-term weight management in adults with obesity or overweight plus a weight-related condition, and for moderate to severe obstructive sleep apnea in adults with obesity.

What is the brand name for tirzepatide?

Tirzepatide is sold by Eli Lilly under two brand names: Mounjaro, approved for type 2 diabetes, and Zepbound, approved for weight management and obstructive sleep apnea. Both contain the same medicine in the same strengths, from 2.5 mg to 15 mg per weekly dose.

What is GIP and GLP-1?

GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are the two main incretin hormones, which the gut releases after eating to help the pancreas release insulin. Tirzepatide switches on receptors for both, while semaglutide switches on only the GLP-1 receptor.

Is tirzepatide stronger than semaglutide?

In the 72-week SURMOUNT-5 trial of 751 adults with obesity and no type 2 diabetes, tirzepatide led to an average weight loss of 20.2%, compared with 13.7% on semaglutide. Both were given at the highest dose each person tolerated.

How long does tirzepatide stay in your system?

The US labels give tirzepatide an elimination half-life of about 5 days (5 to 6 days in people with overweight or obesity), which is why it is taken once a week. Half-life is the time it takes for half of the drug to leave the blood.

Sources

ZEPBOUND (tirzepatide) injection. US prescribing information, sections 1, 3, 6, 11, 12.1, 12.3 and 14 and boxed warning. Eli Lilly and Company. DailyMed, version effective August 28, 2026. DailyMed
MOUNJARO (tirzepatide) injection. US prescribing information, sections 1, 3, 12.1 and 12.3 and boxed warning. Eli Lilly and Company. DailyMed, version effective August 27, 2026. DailyMed
Seino Y, Fukushima M, Yabe D. GIP and GLP-1, the two incretin hormones: Similarities and differences. J Diabetes Investig. 2010;1(1-2):8-23. PubMed · Full text (PMC)
Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). N Engl J Med. 2022;387(3):205-216. PubMed · doi:10.1056/NEJMoa2206038
Aronne LJ, Horn DB, le Roux CW, et al. Tirzepatide as Compared with Semaglutide for the Treatment of Obesity (SURMOUNT-5). N Engl J Med. 2025;393(1):26-36. PubMed · doi:10.1056/NEJMoa2416394
Malhotra A, Grunstein RR, Fietze I, et al. Tirzepatide for the Treatment of Obstructive Sleep Apnea and Obesity (SURMOUNT-OSA). N Engl J Med. 2024;391(13):1193-1205. PubMed · Full text (PMC) · doi:10.1056/NEJMoa2404881

This article summarises published research for general information. It is not medical advice and does not replace advice from your own care team. Never change a medication or dose without talking to your prescriber.