What it is

Tirzepatide is a once-weekly injectable peptide that activates two gut-hormone receptors at once: the GIP receptor and the GLP-1 receptor [1]. Its amino-acid chain carries a C20 fatty diacid that binds to albumin in the blood, which is what stretches its half-life to about 5 days [1]. In the US it is sold by Eli Lilly under two names. Mounjaro, first approved in 2022, is an adjunct to diet and exercise to improve glycemic control in adults with type 2 diabetes [1]. Zepbound is for reducing excess body weight and maintaining that reduction in adults with obesity, or with overweight plus at least one weight-related condition [2]. Both are clear solutions in single-dose pens or single-dose vials [2]. Doses are written in milligrams, which is how STACK logs them.

How it works

GIP and GLP-1 are hormones the gut releases after a meal. Tirzepatide selectively binds and activates both of their receptors [1]. In type 2 diabetes it enhances first- and second-phase insulin secretion and reduces glucagon, and it does both in a glucose-dependent way [1]. GLP-1 is also a physiological regulator of appetite and caloric intake, and nonclinical studies suggest that adding GIP may further reduce food intake [2]. Tirzepatide delays gastric emptying, which can change how quickly other oral medicines are absorbed; the label reports this effect is largest after the first dose and diminishes over time [1]. Exposure is similar whether the injection goes into the abdomen, thigh, or upper arm, and the label says to rotate injection sites with each dose [1].

What the evidence shows

SURMOUNT-1 (NEJM 2022) enrolled 2,539 adults with obesity, or overweight with a weight-related complication, and no diabetes. Participants received 5 mg, 10 mg or 15 mg of tirzepatide once weekly or placebo for 72 weeks; mean weight change at week 72 was -15.0%, -19.5% and -20.9% respectively, versus -3.1% with placebo [3]. SURPASS-2 (NEJM 2021) compared tirzepatide with semaglutide 1 mg in 1,879 adults with type 2 diabetes over 40 weeks; tirzepatide lowered glycated hemoglobin by 2.01 to 2.30 percentage points versus 1.86 with semaglutide and was noninferior and superior [4]. Two 52-week trials in 469 adults with obesity and moderate to severe obstructive sleep apnea showed a statistically significant reduction in the apnea-hypopnea index versus placebo, which FDA says is likely related to weight reduction [6].

Regulatory status

Tirzepatide is an FDA-approved drug. Mounjaro’s initial US approval was in 2022 for type 2 diabetes [1]. On Nov 8 2023 FDA approved Zepbound for chronic weight management in adults with obesity or overweight with a weight-related condition [5]. On Dec 20 2024 FDA approved Zepbound for moderate to severe obstructive sleep apnea in adults with obesity, the first drug treatment for that condition [6]. On May 1 2026 FDA published a Federal Register notice proposing not to include tirzepatide on the 503B bulks list; FDA tentatively found no clinical need for outsourcing facilities to compound from bulk tirzepatide, and comments closed Jun 30 2026 [7]. FDA also reports that, as of May 31 2026, it had received more than 730 adverse event reports tied to compounded tirzepatide [9]. This is the status as of Sep 24 2026.

Half-life and what it means for a level curve

The Mounjaro label gives an elimination half-life of approximately 5 days, measured in adults with type 2 diabetes after subcutaneous injection [1]. The 2024 Zepbound label reports approximately 5 days in adults with overweight or obesity [2]; the current Zepbound label widens that to approximately 5–6 days, which is 120 to 144 hours [8]. STACK uses the lower, Mounjaro value. Peak plasma concentration arrives 8 to 72 hours after an injection [1]. The reference half-life in STACK is 120 hours. Because 5 days is shorter than the 7-day dosing interval, the curve rises after each injection and falls by roughly 60% before the next one, so it looks like a sawtooth that climbs toward a plateau. With regular dosing a level reaches about 97% of steady state after about 5 half-lives, which here is roughly 25 days. STACK’s curve is an estimate from this published half-life, not a blood level.

Reported side effects

In placebo-controlled type 2 diabetes trials, the most common adverse reactions with Mounjaro were nausea (12 to 18% versus 4% on placebo), diarrhea (12 to 17% versus 9%), decreased appetite, vomiting, constipation and dyspepsia [1]. In the Zepbound weight-reduction trials, nausea was reported by 25 to 29% (8% placebo), diarrhea by 19 to 23% (8%), vomiting by 8 to 13% (2%), constipation by 11 to 17% (5%), injection site reactions by 6 to 8% (2%) and hair loss by 4 to 5% (1%) [2]. In SURMOUNT-1, gastrointestinal events were mostly mild to moderate and occurred primarily during dose escalation [3]. Both labels carry a boxed warning: tirzepatide causes thyroid C-cell tumors in rats, human relevance is unknown, and it is contraindicated with a personal or family history of medullary thyroid carcinoma or MEN 2 [1]. Other warnings include pancreatitis, hypoglycemia when combined with insulin or secretagogues, hypersensitivity, and pulmonary aspiration during anesthesia [1]. Report anything unexpected to your prescriber.

Storage and handling

Store Mounjaro and Zepbound in a refrigerator at 2°C to 8°C (36°F to 46°F), in the original carton to protect from light [1]. If needed, a single-dose pen or vial can stay unrefrigerated at up to 30°C (86°F) for up to 21 days [1]. Do not freeze, and do not use a pen or vial that has been frozen [1]. For Zepbound, a pen or vial that has been kept at room temperature should not go back into the refrigerator, and should be discarded if unused within 21 days [2]. Both products are single-dose; opened vials are thrown away after use even if medicine remains [1]. The branded pens and vials are ready-to-use solutions, so no mixing is involved. If your prescriber gave you a product that needs reconstitution, STACK’s reconstitution calculator does the volume arithmetic; it does not suggest amounts.

What STACK tracks for it

STACK logs each tirzepatide dose exactly as your prescriber wrote it, in milligrams, with the date and time. From those entries it draws an estimated level curve using the 120-hour reference half-life. It stores the titration schedule your prescriber gave you so you can see whether you are on plan, without ever proposing one. You can record the injection site for each dose, which supports the label instruction to rotate sites among the abdomen, thigh and upper arm [1]. Weight, side effects and how long they lasted, and pen or vial inventory are tracked alongside, including a 21-day countdown for any pen or vial taken out of the refrigerator [2]. STACK shows your data; it never prescribes.

The reference half-life in STACK

STACK's simulator and the app use 120 h (5 days) for Tirzepatide: approximately 5 days (about 120 h) per the Mounjaro label; the current Zepbound label says approximately 5–6 days (120–144 h). Measured in: adults with type 2 diabetes (Mounjaro label) and adults with obesity or overweight (Zepbound label), subcutaneous.

Tirzepatide

120 h (5 days)

Reference half-life from published pharmacokinetics; editable. Educational only — not medical or dosing advice.

“The apparent population mean clearance of tirzepatide is 0.061 L/h with an elimination half-life of approximately 5 days, enabling once-weekly dosing.” — MOUNJARO (tirzepatide) injection — Full Prescribing Information, §12.3 Elimination (2024 revision). U.S. FDA / Eli Lilly and Company. View source

Simulate Tirzepatide Plan with Tirzepatide

Estimate only. Modelled from the published half-life you selected and the doses you logged — not a measured blood level. Do not use it to make medication decisions.

Free tools for Tirzepatide

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Every URL below was fetched and checked on 2026-09-24. Bracketed numbers in the text link here.

Tirzepatide

What it is

How it works

What the evidence shows

Regulatory status

Half-life and what it means for a level curve

Reported side effects

Storage and handling

What STACK tracks for it

The reference half-life in STACK

Tirzepatide

Free tools for Tirzepatide

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