Drug facts

How tirzepatide works

Reviewed August 2026

What the molecule is¹

The description section states that the product is tirzepatide injection, for subcutaneous use, containing a GIP receptor and GLP-1 receptor agonist. Tirzepatide is based on the GIP sequence, with aminoisobutyric acid at positions 2 and 13, a C-terminal amide, and a lysine residue at position 20 attached to 1,20-eicosanedioic acid via a linker. Its molecular weight is 4813.53 Da.

That last detail matters more than it looks: at nearly 4,900 daltons this is a peptide, not a small molecule, which is the reason every approved presentation is an injection and why there is no approved oral tirzepatide.

The two receptors²

Section 12.1 states that tirzepatide selectively binds to and activates both the GIP and GLP-1 receptors, the targets for native GIP and GLP-1, and that it contains a C20 fatty diacid that enables albumin binding and prolongs the half-life. On what the two receptors contribute, the label is careful: GLP-1 is described as a physiological regulator of appetite and caloric intake, while nonclinical studies are said to suggest the addition of GIP may further contribute to the regulation of food intake. Both receptor types are found in areas of the brain involved in appetite regulation.

Read that hedge as written. The label does not claim the GIP component is the reason for any particular result; it says the animal evidence suggests a contribution. The clinical comparison people usually want from that claim is a separate question, taken up in the trial evidence comparing tirzepatide with semaglutide.

What it does in the body³

Section 12.2 lists the pharmacodynamic effects. Tirzepatide lowers body weight with greater fat mass loss than lean mass loss, and decreases calorie intake, effects the label says are likely mediated by affecting appetite. It stimulates insulin secretion in a glucose-dependent manner and reduces glucagon secretion, and it increases insulin sensitivity, as demonstrated in a hyperinsulinemic euglycemic clamp study in patients with type 2 diabetes after 28 weeks. It also delays gastric emptying, with the delay largest after the first dose and diminishing over time.

Two of those have consequences a reader feels directly. The delayed gastric emptying is the mechanism behind the label’s warnings about oral medicines and contraceptives, and the glucose-lowering is why the label treats insulin and sulfonylureas as a hypoglycemia risk rather than a neutral combination.

Half-life, and why dosing is weekly⁴

The chain runs in one direction: the fatty diacid binds albumin, albumin binding prolongs the half-life, and a half-life of five to six days is what makes a once-weekly injection viable. The label states the first two links itself, in section 12.1. It also reports that exposure increases dose-proportionally, and that the pharmacokinetics are similar between healthy subjects and patients with overweight or obesity.

The four-week wait for steady state is the same interval as the minimum gap between dose increases in the approved dosing schedule. It is also the reason a dose held for less than four weeks has not finished showing what it does — a point that bears directly on what is and is not known about dosing below the label.

How it leaves the body⁴

Tirzepatide is metabolized by proteolytic cleavage of the peptide backbone, beta-oxidation of the C20 fatty diacid, and amide hydrolysis. The primary excretion routes for its metabolites are urine and feces, and intact tirzepatide is not observed in either. The label reports no dosage adjustment for renal impairment, including end-stage renal disease, or for hepatic impairment, with no change in pharmacokinetics observed in studies of both.

What a compounded vial inherits from this⁵

The mechanism described here belongs to the molecule, but the measurements belong to the product they were made on. A compounded vial has no clinical pharmacology program of its own and no standardized concentration, so the numbers above should be read as facts about Mounjaro and Zepbound rather than as guarantees about anything else.