Pharmacodynamics (PD) is the study of how the drug affects the organism, while pharmacokinetics (PK) is the study of how an organism affects a drug; they are the two main branches of pharmacology.

Pharmacodynamics is the study of the biochemical and physiologic effects of drugs. The effects can include those manifested within animals (including humans), microorganisms, or combinations of organisms (e.g., infection).
Pharmacodynamics places particular emphasis on dose/response relationships, that is, the relationships between drug concentration and effect.

Main drug actions:
1. Stimulating action through direct receptor agonism and downstream effects.
2. Depressing action through direct receptor agonism and downstream effects.
3. Blocking/antagonizing action; the drug binds the receptor but does not activate it.
4. Stabilizing action; the drug seems to act neither as a stimulant nor as a depressant (some drugs possess receptor activity that allows them to stabilize general receptor activation).
5. Exchanging/replacing substances or accumulating them to form a reserve.
6. Direct beneficial chemical reaction, as in free radical scavenging.
7. Direct harmful chemical reaction, which might result in damage or destruction of the cells, through induced toxic or lethal damage.

Pharmacodynamics and pharmacokinetics together influence dosing, benefit, and adverse effects.

See reference for more information.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on June 6, 2020.