Pharmacokinetics

Pharmacokinetics (PK) describes the concentration of a drug in the body over time. It is usually split into four processes, abbreviated ADME:

• Absorption. How much of the dose reaches the blood, and how fast. The share is bioavailability.
• Distribution. Where it goes once there: blood, tissue, bound to proteins.
• Metabolism. How it is broken down. For peptides, mostly by enzymes that cut peptide bonds.
• Excretion. How the pieces leave, usually in urine.

Its counterpart is pharmacodynamics, what the drug does to the body.

The parameters on a label

Two more terms appear in trial reports. Cmax is the peak concentration. AUC, area under the curve, is total exposure over a period, and it is the number used to compare doses, routes and formulations.

Reading the GLP-1 numbers

Both drugs have small volumes of distribution, close to the volume of blood plasma and the fluid around cells, because they are tightly bound to albumin and stay mostly in circulation. That binding is engineered: a fatty acid side chain grips albumin, which protects the peptide from enzymes and slows filtering by the kidney. Clearance is slow as a result, and slow clearance with a small volume gives a long half-life.

A long half-life sets two practical facts. First, weekly dosing works. Second, steady state takes weeks: the Zepbound label says steady state was reached after 4 weeks, and the Wegovy label says semaglutide stays in the circulation for about 5 to 7 weeks after the last dose. A missed dose or a dose change takes weeks to show fully.

Linear kinetics

Both labels describe dose-proportional exposure. The Zepbound label says tirzepatide exposure "increases in a dose-proportional manner", and the Wegovy label says steady-state concentrations rose proportionally with doses up to 7.2 mg weekly. In practice, doubling the dose roughly doubles the level in the blood. Not every drug behaves this way; some saturate their own clearance and rise faster than the dose.

Where PK numbers come from

For approved drugs, PK is measured in dedicated studies and across trial populations, and the label gives means and ranges. Individual values vary with body weight, kidney function and other factors. For most research peptides, human PK data is thin or absent. A half-life quoted without a species, a route and a study is not a PK result.

The dose log in the app records each dose with its time, and the protocol planner lays out schedules, so weight and side effects can be read against a timeline that respects how slowly these drugs build up.

Related terms: half-life, steady state, bioavailability, pharmacodynamics. Back to the glossary.