How tirzepatide works, in the words the label uses

This page stays close to what the label states about the molecule and its kinetics, because those few facts explain the dosing rules better than any diagram.

The molecule

Section 11 of the Zepbound label describes tirzepatide as a GIP receptor and GLP-1 receptor agonist based on the GIP sequence, with two aminoisobutyric acid substitutions, a C-terminal amide, and a lysine at position 20 carrying a 1,20-eicosanedioic acid via a linker. That fatty diacid is the design feature that lets the peptide bind albumin and last for days rather than minutes, which is what turns a hormone with a half-life of minutes into a once-weekly injection.

GIP is glucose-dependent insulinotropic polypeptide; GLP-1 is glucagon-like peptide-1. Both are gut hormones released after eating. The label's mechanism section (12.1) describes tirzepatide as an agonist at both receptors. It is the dual action that distinguishes tirzepatide from semaglutide, liraglutide and the other single-receptor GLP-1 agonists; the head-to-head numbers are on the tirzepatide vs semaglutide page. The label does not claim to know how much of the effect comes from each receptor, and neither should anyone else.

The kinetics that set the schedule

Section 12.3 gives four numbers worth remembering.

Peak plasma concentration comes at a median of 24 hours after a subcutaneous dose, with a range of 8 to 72 hours. So the day after your injection is the day the drug level is highest, which is also the day people most often notice nausea early in treatment.

The elimination half-life is approximately 5 days. After a week, more than a third of the previous dose is still circulating. That is what makes weekly dosing work and it is also why a dose a few days late is not a restart, the basis of the 96-hour missed-dose rule.

Steady state is reached after 4 weeks of once-weekly dosing. At any given dose, the drug level in your blood keeps climbing for roughly four injections before it levels off. A prescriber who moves you up after four weeks is judging your response to the full effect of the current dose, not a partial one. That is the pharmacological reason the label's minimum hold at each step is four weeks and not two.

Exposure increases in proportion to dose. Doubling the dose roughly doubles the drug level; there is no ceiling effect in absorption across the approved range.

What the kinetics mean for stopping

The same half-life works in reverse. Within two to three weeks of a last dose, drug levels are low; within five weeks, negligible. The clinical consequence is described in SURMOUNT-4 (PubMed 38078870): people who had lost 20.9% of their weight over 36 weeks on tirzepatide and were then switched to placebo regained, on average, 14.0% from week 36 to week 88, while those who continued lost a further 5.5%. The drug does not change anything permanently; it acts while it is present. The stopping and maintenance page has the full numbers.

What this page does not claim

It does not explain why some people lose 30% and others 5%, because the label and the trials do not. It does not compare tirzepatide's receptor activity to other molecules in binding terms, because that is not on the label. And it says nothing about compounded tirzepatide's behaviour in the body, because compounded products are not FDA approved, are not studied in these trials, and are not interchangeable with Zepbound or Mounjaro; the kinetics above are for the approved products. For a longer introduction that covers the trial history and the practical basics, the FormBlends tirzepatide introduction is the starting point.