Drug interactions occur when the effect and/or concentration of a drug is modified by another substance, including a concomitant treatment, over-the-counter medication. Drug interactions can be classified as pharmacokinetic or pharmacodynamic in nature. A pharmacokinetic drug interaction occurs when a drug alters the absorption, distribution, metabolism or excretion of another drug. Pharmacodynamic interactions occur when two drugs act on interrelated receptor sites resulting in either additive or antagonistic effects. The liver is the primary site for metabolism of psychotropic drugs. The processing and elimination of drugs from the body occurs through two phases of metabolism: phase 1 reactions, during which CYP 450 enzymes convert the parent compound to metabolites through processes of oxidation, reduction, or hydrolysis and phase 2 reactions, which couple the metabolites with endogenous substances rendering them more water-soluble for excretion from the body. Drug-drug interactions involving psychotropics are initiated primarily by the CYP family of enzymes, and these drugs and metabolic enzymes can interact in a variety of ways . Regarding antidepressant metabolism, the CYP 2D6 and CYP 3A4 systems may be the two most important metabolic pathways, as most antidepressants are eliminated via these pathways. Clinically significant drug interactions are defined as events in which the safety or effectiveness of one medication is substantially altered by another drug or substance. These interactions can result in potentially dangerous, and sometimes fatal, adverse events. In addition, drug interactions can result in reduced effectiveness of treatment or can lead to adverse events that, although not serious, are bothersome for patients. There are numerous known interactions involving psychotropic drugs. Many interactions have minor clinical significance; however, there are many potentially hazardous interactions that should always be considered. The significance of a drug interaction can also vary between individuals depending on factors such as comorbidities, gender or age. Drug interactions with psychotropics can result in poor tolerability or reduced efficacy, or both, which can negatively impact patient outcomes. Clinicians can improve outcomes for patients by considering the potential for drug-drug interactions when selecting a specific psychotropic, and when evaluating patient progress, compliance, and the incidence of adverse events throughout the course of treatment.