posology

Posology is the study of dosage: how much of a drug to give, how often, and for how long. It is the recipe-writing of medicine. The drug may be excellent, but if the amount or timing is wrong, the result is too little effect, too much toxicity, or both. Posology is the discipline of getting that amount right.

Good dosing balances the pharmacokinetics of the drug, how the body absorbs and eliminates it, against its pharmacodynamics, how effect and harm rise with concentration. From these come practical choices: the loading dose to fill the body quickly, the maintenance dose to keep concentration steady, and the dosing interval that determines how concentration rises and falls between doses.

Posology must be individualized. The same drug needs different doses in a child, an older adult, a pregnant patient, or someone with failing kidneys or liver, because these change how the drug is handled or how the body responds. This is why dosing tables give ranges and why clinicians titrate, adjusting the dose up or down to each patient's response rather than applying a single number to everyone.