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

The substances of interest include any chemical xenobiotic (chemical substance found within an organism that is not naturally produced or expected to be present within the organism), such as pharmaceutical drugs, pesticides, food additives, cosmetics, etc.
Pharmacokinetics describes how the body affects a xenobiotic after administration through the mechanisms of absorption and distribution, metabolic changes, and the effects and routes of excretion of the metabolites of the substance.

To simplify conceptualization of the many processes that take place in the interaction between an organism and a chemical substance, these compartments of a model are used:
1. Liberation: the process of release of a drug from the pharmaceutical formulation.
2. Absorption: the process of a substance entering the blood circulation.
3. Distribution: the dispersion or dissemination of substances throughout the fluids and tissues of the body.
4. Metabolism (or biotransformation, or inactivation): the recognition by the organism that a foreign substance is present and the irreversible transformation of parent compounds into daughter metabolites.
5. Excretion: the removal of the substances from the body.

Pharmacokinetic metrics
• Dose: amount of drug administered.
• Dosing interval: time between drug dose administrations.
• Cmax: the peak plasma concentration of a drug after administration.
• Tmax: time to reach Cmax.
• Cmin: the lowest concentration that a drug reaches before the next dose is administered.
• Volume of distribution: the apparent volume in which a drug is distributed.
• Concentration: amount of drug in a given volume of plasma.
• Absorption half-life: the time required for the concentration of the drug to double its original value for oral and other extravascular routes.
• Absorption rate constant: the rate at which a drug enters into the body for oral and other extravascular routes.
• Elimination half-life: the time required for the concentration of the drug to reach half of its original value.
• Elimination rate constant: the rate at which a drug is removed from the body.
• Infusion rate: rate of infusion required to balance elimination.
• Area under the curve: the integral of the concentration-time curve.
• Clearance: the volume of plasma cleared of the drug per unit time.
• Bioavailability: the systemically available fraction of a drug.
• Fluctuation: peak trough fluctuation within one dosing interval at steady state.

Pharmacokinetics and pharmacodynamics together influence dosing, benefit, and adverse effects.

See reference for more information.

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