Biotransformation of drugs to polar metabolites is a key step in the termination of action and the elimination of drugs used in therapeutics. A large number of factors, including infection, alter the basal rates of drug metabolism and cause a number of untoward effects and drug interactions. Although a number of techniques have been attempted, there appears to be no easy way to determine the capacity of an individual to metabolize a specific drug and to tailor drug dosage to that individual.