What half-life describes
A drug's half-life is the time required for its plasma concentration to fall by 50 percent. For most medicines, elimination follows first-order kinetics, meaning a constant fraction (not a constant amount) is cleared per unit time. Starting from a peak, the concentration drops to 50 percent after one half-life, 25 percent after two, 12.5 percent after three, and so on, producing the characteristic exponential decay curve.
The five half-life rule
By convention, a drug is considered effectively eliminated after about five half-lives, when roughly 97 percent has cleared. The same arithmetic works in reverse for repeated dosing: it takes about five half-lives of regular administration to approach steady-state concentration. Half-life therefore guides dosing interval and how long effects or interactions persist after stopping.
Half-life varies with liver and kidney function, age, drug interactions, and genetics, so population values are estimates. This is general pharmacology for education, not dosing advice; medication timing and discontinuation should be directed by a prescriber or pharmacist.