Pharmacokinetics & dynamics (PK/PD)

If you want to understand how a drug works in the body, these two terms, which belong together, can help:

• Pharmacokinetics (PK): what the body does to the substance: absorption, distribution, metabolism and excretion. This is where half-life (the time it takes the body to break down half of the active ingredient) and bioavailability (how much of the active ingredient actually reaches the bloodstream and can take effect) live.
• Pharmacodynamics (PD): what the substance does to the body: the action at the receptor (that is, at a kind of docking site on cells where the active ingredient latches on and triggers something) and the biological effect.

Rule of thumb

PK = the substance's journey through the body; PD = the substance's effect. Together they explain why a dose acts as strongly and as long as it does.