What is the difference between pharmacokinetics and pharmacodynamics?

• A. Pharmacokinetics describes how the body absorbs, distributes, metabolizes, and excretes a drug (ADME); pharmacodynamics describes the drug's biological effects at target receptors and the dose-response relationship.
• B. Pharmacokinetics describes the drug's effects on targets; pharmacodynamics describes ADME.
• C. Pharmacokinetics is about drug development; pharmacodynamics is about regulatory approval.
• D. Pharmacokinetics concerns only metabolism; pharmacodynamics concerns only receptors.

Answer: (A)

The key idea is that pharmacokinetics answers how the body handles a drug, while pharmacodynamics explains what the drug does to the body. Pharmacokinetics describes processes that determine drug concentrations over time: absorption into the bloodstream, distribution to tissues, metabolism into metabolites, and excretion (the ADME sequence). These factors tell you how quickly a drug acts, how long it stays active, and what dose is needed to reach therapeutic levels. Pharmacodynamics, in contrast, looks at the drug’s effects once it reaches its target: how it interacts with receptors or other targets, the magnitude of the response, and how the response changes with different doses (the dose–response relationship). So PK is about the journey and fate of the drug in the body, and PD is about the biological effects and how strong they are at various doses. The other statements invert these roles or bring in unrelated topics like development or regulatory approval, which don’t capture the actual distinction between PK and PD.