Definition: Pharmacokinetics (PK) is what the body does to a drug: absorbing it, spreading it around, breaking it down and getting rid of it. Pharmacodynamics (PD) is what the drug does to the body: the effect it causes, usually by acting on a receptor. They are the two branches of pharmacology.
The two halves
StatPearls sums up PK as four steps, often shortened to ADME: absorption, distribution, metabolism and excretion. Terms like half-life, Cmax, clearance and volume of distribution all describe PK.
PD covers what happens once the drug reaches its target. StatPearls lists the main kinds of action: switching a receptor on (an agonist), blocking it without switching it on (an antagonist), and direct chemical reactions. Blood sugar falling after insulin is a PD effect.
Worked example: level and effect on two labels
PK and PD numbers side by side
- Macrilen (macimorelin) PK: the label says the peak blood level (Cmax) came 0.5 to 1.5 hours after an oral dose, taken fasting.
- Macrilen PD: the label says the highest growth hormone levels show up 30 to 90 minutes after the dose. Here the drug level and the effect peak at about the same time.
- Egrifta SV (tesamorelin) PK: the label gives a mean half-life of 8 minutes. 8 minutes × 5 half-lives = 40 minutes until most of a dose is gone.
- Egrifta SV PD: the label is dosed once a day and asks for IGF-1 levels to be monitored. The marker people watch is the hormone response, not the drug level.
These are averages from study groups. Individual numbers vary with body size, kidney and liver function and the product itself.
Why it matters when you track doses
A log of dose times is PK data. A log of how you feel, your weight or a lab value is PD data. Keeping both lets you see whether an effect follows the dose, lags behind it, or outlasts it. A short half-life does not always mean a short effect.
Read more on Dose Track: Half-life calculator · Tesamorelin guide
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
What is the difference between PK and PD?+
PK (pharmacokinetics) describes how the body absorbs, spreads, breaks down and removes a drug. PD (pharmacodynamics) describes the effect the drug has on the body.
Is half-life PK or PD?+
Half-life is a PK measure. It tracks how fast the drug level falls, not how long the effect lasts.
Can an effect last longer than the drug level?+
It can. Tesamorelin has an 8 minute half-life on its label but is dosed once a day, and the label tracks its effect through IGF-1 levels.