Drugs administered to patients are biotransformed by a number of drug metabolizing enzymes. Considerable parts of drugs as well as environmental chemicals are metabolized by the hepatic cytochrome P450 system. Drug interactions of metabolism have varying clinical relevance. Some of the drugs can induce their own metabolism and also affect the metabolism of other drugs. Similarly, drugs metabolized by the same enzyme system can act as competitive inhibitors and decrease the metabolism of other drugs. Furthermore, some drugs can inhibit other drugs through non-competitive manner. There has been much improvement on the pharmacogenetics that deal with variability in drug response due to heredity. It is now known that there is a great deal of individual variability in the metabolic capacity of the hepatic enzyme systems and the degree to which it may be induced or inhibited by drugs. Although it is not easy to predict the extent of an interaction in an individual patient, optimum drug therapy may be accomplished by understanding the mechanism of enzyme induction or inhibition and the principle of pharmacogenetics as well.