Pharmacokinetics

There are two halves to how a drug and a body interact. Pharmacokinetics is one of them: what the body does to the drug. (The other half, what the drug does to the body, is pharmacodynamics.)

The classic way to remember it is four stages, sometimes called ADME: Absorption (getting in), Distribution (spreading through the body), Metabolism (getting broken down), and Elimination (getting out). Together they describe the whole journey of a dose from the moment you take it to the moment it’s gone.

An everyday parallel: track a delivery from the warehouse. When did it arrive, where did it end up, how long did it sit, when was it thrown out? Pharmacokinetics is that tracking log for a molecule.

Why it matters for peptide research: pharmacokinetics is where half-life, bioavailability, and peak levels all live. It’s what tells you whether a compound needs daily dosing or weekly, why an injection works when a pill wouldn’t, and how long it lingers. When a study reports “peak concentration at 90 minutes” or “cleared within 24 hours,” that’s pharmacokinetics — the practical numbers behind how and when to dose.

For a concrete example, semaglutide shows all four ADME stages in one compound. After a subcutaneous injection, its prescribing information reports 89% absolute bioavailability and a peak blood concentration one to three days later; once absorbed, more than 99% binds to albumin, a blood protein that helps carry it through circulation and slows its clearance. The body then breaks down the peptide backbone and fatty-acid side chain, with related material leaving through urine and feces, while only about 3% of the dose is excreted in urine as intact semaglutide. Put together, those measurements explain the practical result: an elimination half-life of about one week and a once-weekly schedule, rather than a vague claim that the compound simply “lasts a long time.”