Drug interactions occur, when the effect and/or the metabolism of a drug is altered by another drug or substance. A drug interaction can be beneficial, damaging, or have no significant effect. The majorities of drug interactions have minor clinical significance but sometimes can result in potentially hazardous, and occasionally lethal adverse events. Drug interactions can be classified as pharmacokinetic or pharmacodynamic. Pharmacokinetic interactions occur when a drug modifies the absorption, distribution, metabolism or excretion of another co-administered drug. These interactions are mainly a result of inhibition or induction of the cytochrome P450 (CYP450) isozymes. Pharmacodynamic interactions occur when the concurrent use of two drugs results in an alteration of the therapeutic and/or toxic effects of either drug without altering their pharmacokinetics, and can be additive, synergistic or antagonistic. The pharmacokinetics of a drug defines its potential for drug-drug interactions. Most drug interactions with psychotropics are pharmacokinetic and include the CYP450 family of enzymes. Drug interactions with psychotropics can cause adverse effects due to inhibition of CYP450 enzyme or can reduce therapeutic efficacy duo to induction of CYP450 enzyme. There is also a large inter-individual variation in the amount of isoenzymes that individuals possess; different individuals can be poor metabolizers or ultrafast metabolizers with respect to particular CYP450 enzymes. However, not all drug-drug interactions via the CYP enzyme system are of clinical significance. Additionally, a clinician must consider significant other factors to the patients. Younger people tend to metabolize drugs faster than older people, men faster than women. Comorbid medical conditions and gene polymorphism may also affect drug metabolism. Dose reductions in special populations may be necessary when the factors affecting plasma clearance are present. Assessment of the pharmacodynamic and pharmacokinetic profile of psychotropic and other drugs can help determine the clinical significance of any interactions and prevent the serious consequences of drug interactions.