A drug interaction, defined as the modification of the action of one drug by another, can be beneficial or harmful, or it can have no important effect. An acknowledgment of drug interactions at the clinics is becoming increasingly necessary with the rising use of combinations of drugs in all medical conditions. Drug interactions are usually classified as pharmaceutical, pharmacodynamic and pharmacokinetic. Of the three mechanisms, pharmaceutical interactions are least likely to lead to problems in clinical practice and there are no potentially risky interactions of this type with psychotropic drugs. Pharmacokinetic interactions occur when the absorption, distribution or elimination of one drug is inşuenced by another. The most common interactions seen in clinical practice are pharmacodynamic. They occur when drugs compete for the same receptor or produce antagonistic or synergistic effects on the same target organ or system. The most important enzymes involved in drug interactions are members of the cytochrome P450 (CYP) system that are responsible for many of the phase 1 biotransformations of drugs. The other potential for interactions involving uridine diphosphate glucuronosyl transferases (UGT) responsible for phase two conjugation reactions, is newly recognized very well. Many psychotropic drugs have a high affinity for one or more of the enzymes in the CYP or UGT systems, which play a major role in their metabolism. Any attempt to keep hundreds of potential drug interactions in mind to prevent hazardous interactions is ineffective. Rather, a child psychiatrist should have a basic understanding of the types and timing of possible drug interactions and then develop prevention strategies in prescribing psychotropics. Additionally, drug interaction simulation software that detail both reported and predicted CYP-based, glucuronidation-based and other drug interactions are emerging to be part of a clinician armamentarium. Finally, two cases will be discussed about this topic in this section.