Tirzepatide (Mounjaro, Zepbound): What It Is, How It Works and What the Label Says

Short answer: tirzepatide is a once-weekly injection from Eli Lilly that activates two hormone receptors, GIP and GLP-1. It is sold as Mounjaro, for type 2 diabetes since May 13, 2022, and Zepbound, for chronic weight management since November 8, 2023 and for obstructive sleep apnea with obesity since December 20, 2024.^[1][2][3] In SURMOUNT-1, body weight fell by an average of 20.9% over 72 weeks on 15 mg, against 3.1% on placebo.^[3][4] Both carry a boxed warning about thyroid C-cell tumors.^[3][2]

New to eating less? Start with what to eat when you're eating less, the guide the rest of Lighter Days builds on.

This is our reference page for tirzepatide, written from the FDA prescribing information as it stands on November 13, 2026: which product is approved for what and since when, how the drug works, the headline SURMOUNT and SURPASS trials, the label's warnings, what Zepbound vials and pens cost through LillyDirect, and where to go next on eating. Semaglutide, the single-receptor medicine behind Ozempic and Wegovy, has its own page: semaglutide (Ozempic, Wegovy, Wegovy pill).

General information, not medical advice. Nothing here tells you whether to start, stop or change a medication or a dose. That is a decision for you and your prescriber.

Tirzepatide at a glance

Brand and form	FDA-approved uses (date approved)	How it's taken	Label dose range
Mounjaro, Eli Lilly: single-dose pen, vials or multi-dose KwikPen	Blood sugar control in adults with type 2 diabetes (May 13, 2022) and in children 10 and older (December 19, 2025); lowering the risk of heart attack, stroke and cardiovascular death in adults with type 2 diabetes at high cardiovascular risk (August 27, 2026)[1][2]	Once a week, under the skin	Starts at 2.5 mg; raised in 2.5 mg steps; maximum 15 mg in adults, 10 mg in children[2]
Zepbound, Eli Lilly: single-dose pen, vials or multi-dose KwikPen	Chronic weight management in adults with obesity, or overweight plus a weight-related condition (November 8, 2023); moderate to severe obstructive sleep apnea in adults with obesity (December 20, 2024)[1][3]	Once a week, under the skin	Starts at 2.5 mg; maintenance 5, 10 or 15 mg (10 or 15 mg for sleep apnea); maximum 15 mg[3]

Also on both labels	What it says
Boxed warning	Risk of thyroid C-cell tumors. Tirzepatide caused them in rats; whether it does in humans is unknown. Not for anyone with a personal or family history of medullary thyroid carcinoma or with MEN 2.[3][2]
Most common adverse reactions, Zepbound 5 / 10 / 15 mg vs placebo	Nausea 25% / 29% / 28% vs 8%; diarrhea 19% / 21% / 23% vs 8%; vomiting 8% / 11% / 13% vs 2%; constipation 17% / 14% / 11% vs 5%; abdominal pain 9% / 9% / 10% vs 5%; dyspepsia 9% / 9% / 10% vs 4%; injection site reactions 6% / 8% / 8% vs 2%[3]
Most common, Mounjaro 5 / 10 / 15 mg vs placebo (type 2 diabetes)	Nausea 12% / 15% / 18% vs 4%; diarrhea 12% / 13% / 17% vs 9%; decreased appetite 5% / 10% / 11% vs 1%; vomiting 5% / 5% / 9% vs 2%; constipation 6% / 6% / 7% vs 1%[2]

The Zepbound and Mounjaro figures come from different trials in different people, so they are not a ranking. The labels say rates from one drug's trials cannot be compared directly with another's.^[3]

What is tirzepatide?

Tirzepatide is a single molecule that activates the receptors for two gut hormones: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). It carries a fatty side chain that binds to albumin in the blood, which stretches its half-life to once-weekly dosing.^[3] That two-receptor design is why it is sometimes called a "dual agonist", and why it sits between semaglutide (GLP-1 only) and the investigational retatrutide (GIP, GLP-1 and glucagon), covered in GLP-3 (retatrutide).

Mounjaro and Zepbound contain the same active ingredient at the same strengths, 2.5 mg to 15 mg.^[2][3] The difference is the approved use, and with it insurance coverage and price. The Zepbound label says not to use it with other tirzepatide-containing products or any GLP-1 receptor agonist.^[3]

How does tirzepatide work?

• Appetite. GLP-1 is a natural regulator of appetite and calorie intake; nonclinical studies suggest GIP may add to the regulation of food intake. Both receptors are found in brain areas involved in appetite, and in animal studies tirzepatide reached and activated neurons there. The label says it lowers calorie intake, likely by affecting appetite.^[3]
• Blood sugar. It increases insulin release in a glucose-dependent way, lowers glucagon, and improved insulin sensitivity in a 28-week clamp study in people with type 2 diabetes.^[3]
• Slower stomach emptying. Tirzepatide delays gastric emptying. The delay is largest after the first dose and lessens over time, the label says.^[3]
• Body composition. It lowers body weight with more fat mass lost than lean mass.^[3] Lean mass still falls, which is the reason for the protein and strength-training advice in the eating section.

What did the tirzepatide trials find?

The weight-management trials are called SURMOUNT and the diabetes trials SURPASS. Body weight results are average percent change from starting weight, counting everyone randomized.

Trial	Who took part	Length and dose	Main result
SURMOUNT-1 (Zepbound)	2,539 adults with obesity, or overweight plus a related condition; no diabetes	72 weeks; 5, 10 or 15 mg	Body weight −15.0%, −19.5% and −20.9% vs −3.1% on placebo[3][4]
SURMOUNT-2 (Zepbound)	938 adults with type 2 diabetes and a BMI of 27 or more	72 weeks; 10 or 15 mg	Body weight −12.8% and −14.7% vs −3.2%[3]
SURMOUNT-OSA (Zepbound)	469 adults with obesity and moderate to severe sleep apnea, in two trials: not using PAP, and using PAP	52 weeks; 10 or 15 mg	Breathing interruptions fell by 25.3 and 29.3 events an hour, vs 5.3 and 5.5 on placebo, from starting averages near 50[5][3]
SURPASS-1 (Mounjaro)	478 adults with type 2 diabetes on diet and exercise alone	40 weeks; 5, 10 or 15 mg	HbA1c fell 1.7 to 1.8 points vs 0.1 on placebo[2]
SURPASS-2 (Mounjaro vs semaglutide)	1,879 adults with type 2 diabetes on metformin	40 weeks; 5, 10 or 15 mg vs semaglutide 1 mg	HbA1c −2.01, −2.24 and −2.30 points vs −1.86; body weight 1.9, 3.6 and 5.5 kg lower than with semaglutide[6]
SURPASS-CVOT (Mounjaro vs dulaglutide)	13,299 adults with type 2 diabetes and heart disease	Median about 4 years; up to 15 mg vs dulaglutide 1.5 mg	Heart attack, stroke or cardiovascular death in 12.2% vs 13.1% (hazard ratio 0.92): not worse than dulaglutide, but superiority was not shown[7][2]

Three notes on reading the table. SURPASS-2 compared tirzepatide with semaglutide 1 mg, the Ozempic diabetes dose, not with Wegovy 2.4 mg, so it is not a test of the two weight-management products.^[6] SURPASS-CVOT used an active comparator that itself lowers cardiovascular risk, which is why its result reads as "non-inferior" rather than as a placebo comparison.^[2] And in SURMOUNT-1, 4.8% to 6.7% stopped tirzepatide because of an adverse reaction, against 3.4% on placebo, mostly in the first few months and mostly for digestive reasons.^[3] The trials also coached a deficit of about 500 calories a day and at least 150 minutes of activity a week.^[3]

Tirzepatide dosing: what the FDA label says

Both labels raise the dose in 2.5 mg steps, no faster than every 4 weeks, to reduce digestive reactions.^[3][2] The Zepbound schedule is reproduced here so you can see what the label says, not so you can follow it yourself.

What the FDA label says — not dosing advice; your prescriber sets your dose. Zepbound, once weekly.^[3]

Step	Weekly dose
Weeks 1 to 4	2.5 mg (starting dose only; not a maintenance dose)
From week 5	5 mg
After at least 4 weeks on the current dose	May be raised by 2.5 mg: 7.5 mg, 10 mg, 12.5 mg, 15 mg
Maintenance	5, 10 or 15 mg for weight management; 10 or 15 mg for sleep apnea
Maximum	15 mg

Mounjaro follows the same steps for diabetes, up to 15 mg in adults and 10 mg in children; since August 2026 its label also allows 2.5 mg as an ongoing dose for blood sugar control.^[2][1] Unlike the semaglutide tablets, the injections have no food rule: the label says they can be taken at any time of day, with or without meals.^[3]

What does the tirzepatide label warn about?

The Zepbound label lists these warnings and precautions, and Mounjaro's are the same set.^[3][2]

• Thyroid C-cell tumors in rats; human relevance not determined. Report a lump in the neck, trouble swallowing, shortness of breath or persistent hoarseness.^[3]
• Severe digestive reactions, in 1.7% to 3.1% by dose vs 1% on placebo. Not recommended with severe gastroparesis.^[3]
• Acute kidney injury from dehydration, mostly after vomiting, diarrhea or nausea.^[3] Signs to watch are in dehydration on a GLP-1.
• Gallbladder disease. Inflammation of the gallbladder in 0.7% vs 0.2% on placebo.^[3]
• Acute pancreatitis, including fatal cases with GLP-1 medications.^[3]
• Serious allergic reactions, including anaphylaxis and angioedema.^[3]
• Low blood sugar, reported in 4.2% vs 1.3% in people with type 2 diabetes, and in 10.3% of those also taking a sulfonylurea.^[3]
• Diabetic eye disease can temporarily worsen with fast improvement in blood sugar.^[3]
• Breathing in stomach contents under anesthesia, reported rarely with GLP-1 medications. Tell the surgical team you take it.^[3]
• Birth control pills. Because stomach emptying slows, the label advises people on oral hormonal contraceptives to switch to a non-oral method, or add a barrier method, for 4 weeks after starting and for 4 weeks after each dose increase.^[3]

Low blood pressure was reported in 1.6% vs 0.1% on placebo, more often with blood pressure medicines,^[3] and hair loss in 4% to 5% vs 1%, which we cover in does Zepbound cause hair loss. Constipation has its own page, Zepbound and constipation, and tiredness is in does Zepbound make you tired.

How much does tirzepatide cost, and what are Zepbound vials?

Zepbound comes in three forms: a single-dose pen, single-dose vials, and a multi-dose KwikPen holding 4 doses.^[3] Lilly says the vials and multi-dose KwikPens are sold only through LillyDirect's self-pay pharmacy channel, with a prescription.^[8] Lilly's own prices as of November 13, 2026, for people paying without coverage:^[9]

• $299 a month for 2.5 mg, as a KwikPen or 4 vials.
• KwikPen: $399 a month for 5 mg, and $449 for 7.5 mg to 15 mg under a purchase offer that requires refilling on time; the regular price is $499 for 7.5 mg and $699 for 10 to 15 mg.
• Single-dose pen: as little as $25 with commercial insurance that covers it, or as low as $499 a month with commercial insurance that does not, under Lilly's savings card, which ends December 31, 2026.
• Medicare: Lilly lists Zepbound coverage for weight management through the Medicare GLP-1 Bridge at no more than $50 a month for eligible Part D patients.

Self-pay prices are set by Lilly and change often; check them before you rely on them. A full cost comparison across brands is coming as its own page. Compounded tirzepatide is not an FDA-approved product, and what that means for quality and legality is in compounded GLP-1s explained.

What should you eat on tirzepatide?

The Zepbound label pairs it with a reduced-calorie diet and more activity but names no diet.^[3] The 2025 joint advisory from four US medical and nutrition societies sets the priorities: protein first, small nutrient-dense meals, fiber from food, deliberate fluids, and strength training to protect lean mass.^[10] Our pages go one question at a time:

• What to eat on Zepbound: a small-appetite day, with amounts.
• Foods to avoid on a GLP-1, and what to eat on queasy days.
• How much protein on a GLP-1 and keeping muscle on a GLP-1.
• How much water to drink, plus coffee and alcohol on a GLP-1.
• The whole map: our GLP-1 nutrition guide.

Related medicines

Semaglutide (Ozempic, Wegovy, the Wegovy pill) acts on GLP-1 alone and has its own reference page: semaglutide. Retatrutide, Lilly's investigational "triple agonist" nicknamed GLP-3, is not approved; where it stands is in GLP-3 (retatrutide).

For transparency about why this page is on our site: we make one product, Zornava Replenish, a lemon-lime daily hydration and nutrient powder for lighter appetite days, including for people on GLP-1 medications. It is a dietary supplement, not a medication, and it is not a substitute for anything a prescriber recommends. It is not on sale yet; early access opens February 9, 2027, and the early-access list is open now.