Duration of drug action depends on several factors: the absolute amount of drug given; the pharmaceutical preparation; the reversibility of drug action; the half-life of the drug; the slope of the concentration-response curve; the activity of metabolites, and the influence of disease on drug elimination. The duration of action of a drug with more than one effect may differ, depending on which effect is followed. Drugs with a prolonged therapeutic effect should be given once or twice daily. Drugs with a short duration of therapeutic effect are best given at regular intervals rather than by t.i.d. or q.i.d. regimens.