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A drug interaction occurs when the effectiveness or toxicity of a drug is modified by the addition of another drug. Sometimes, drug interactions may be beneficial, leading to increased efficacy or reduced risk of side effects. However, more often, drug interactions are important when concurrent drug administration leads to decrease in efficacy or increase in toxicity of medications. Drug-drug interactions occur via two ways; pharmacodynamic and pharmacokinetic interactions. Pharmacodynamic interactions mean synergy or antagonism of each drug’s effects at target receptors. Pharmacokinetic drug-drug interactions may occur by mechanisms, including alterations in drug metabolism, absorption, excretion, and distribution. In pharmacokinetic interactions, blood level of administered drug changes. Many psychotropic drugs have significant interactions with other medications. Psychotropic drugs are mostly metabolized in the liver by cytochrome P450 (CYP) enzymes. Therefore, they are sensitive to metabolically based drug interactions. Psychotropic drug-drug interaction may occur with other psychotropic medications as well as with agents used for the treatment of accompanying somatic illnesses. P450 inhibitors produce an increase in blood levels of drugs, which are metabolized by CYP enzymes. Conversely, inducers cause an increase in metabolization and decrease in blood levels. Enzymatic inhibition is usually immediate, whereas induction requires several days to two or more weeks to show an effect on drug metabolism. In child psychiatry practice, antipsychotics (especially risperidone, aripiprazole and olanzapine), selective serotonin reuptake inhibitors (şuoxetine, sertraline, and escitalopram), tricyclic antidepressants (imipramine), methylphenidate and atomoxetine are mostly prescribed. CYP2D6, CYP3A4 and CYP2C9 are commonly involved in psychotropic metabolism. Therefore, other drugs prescribed for somatic diseases altering CYP enzymes can change psychotropic drug levels. In summary, risperidone is metabolized primarily by CYP2D6 and, to a lesser extent, CYP3A4. Aripiprazole is metabolized by CYP2D6 and CYP3A4. Fluoxetine and paroxetine are potent inhibitors of CYP2D6; sertraline inhibits CYP3A4. Stimulants react with other drugs through pharmacodynamic reactions. Dopaminergic, noradrenergic and possible serotonergic receptors play role in stimulant and other drug interactions. Atomoxetine does not alter CYP enzymes but drugs affecting on CYP2D6 can cause differences in atomoxetine metabolism. Carbamazepine stimulates many enzymes especially CYP3A4 and leads to many important alterations. It is essential to be aware of any drug, prescription or otherwise, that the patient may be taking concurrently and to evaluate the potential interaction. As part of medical history, enquiries should be made about all medications, including those prescribed by other physicians; over-the-counter drugs (Aspirin, painkiller); dietary supplements (ginseng, grapefruit juice); and herbal preparations (St John’s worth) and alcohol use. Drugs used for common childhood disorders such as asthma (theophylline, beta agonists), infections (erythromycin), epilepsy (e.g. carbamazepine, phenytoin), fungal infections (e.g. şuconazole), nasal congestion (pseudoephedrine) can cause drug-drug interactions with psychotropics. Therefore, clinicians must be careful in clinical practice Two case scenario about psychotropic drug interactions with non-psychotropic medications will be discussed in the presentation.