[Author Affiliation]Jennifer L. Kruse. 1 Department of Psychiatry & Biobehavioral Sciences, David Geffen School of Medicine at UCLA, Los Angeles, California.Michael C. Davis. 1 Department of Psychiatry & Biobehavioral Sciences, David Geffen School of Medicine at UCLA, Los Angeles, California. 2 VA Greater Los Angeles Mental Illness Research, Education and Clinical Center, Los Angeles, California.Address correspondence to: Jennifer L. Kruse, MD, David Geffen School of Medicine at UCLA, Department of Psychiatry and Biobehavioral Sciences, 300 UCLA Medical Plaza, Los Angeles, CA 90095, E-mail: jkruse@mednet.ucla.eduTo The Editor:Treatment of psychiatric illness with more than one medication is common; this places patients at risk for drug-drug interactions. Altered metabolism resulting from drug interactions may lead to greater drug effects caused by higher serum levels of a drug, or reduced efficacy caused by lower serum levels of a drug, depending upon the interaction. We present a case of a child on a stable dose of risperidone, who developed new-onset extrapyramidal symptoms upon tapering and discontinuing oxcarbazepine.Case ReportAn 11-year-old boy with past psychiatric diagnoses of bipolar disorder, generalized anxiety disorder, autism spectrum disorder, and attention-deficit/hyperactivity disorder, was admitted to inpatient psychiatry for increasing aggression and violence toward family members and animals. History gathered from the patient and his family revealed the absence of any discrete mood episodes that would meet criteria for mania, hypomania, or depression. Developmental history was unremarkable; he met all developmental milestones on schedule. The patient demonstrated the ability to socialize appropriately and to make friends easily both in school and in an inpatient psychiatric facility. He was well behaved and attentive in the inpatient unit, without behavioral disturbances. During his hospitalization, the diagnoses of bipolar disorder and autism spectrum disorder were ruled out following detailed assessment, including detailed clinical interviews with the patient and his parents, as well as behavioral observations from experienced psychiatric nurses. Conduct disorder was diagnosed based on a history of violence toward people and animals, destruction of property, and deceitfulness.At the time of admission, his psychotropic medications included risperidone 2 mg twice daily and oxcarbazepine 600 mg twice daily. He had been treated with these medications since age 9, and had been on a stable dosage of both medications for 1 month preceding hospital admission. During his inpatient stay, oxcarbazepine was tapered and discontinued over the course of 1 week, given lack of clear indication, as well as lack of beneficial effects in the outpatient setting. The patient tolerated the taper without difficulty.One day following completion of the oxcarbazepine taper, the on-call psychiatrist was paged to evaluate the patient for new onset of mild dysarthria. Additionally, the patient had a subjective complaint of a funny feeling in his tongue. Examination for extrapyramidal symptoms was unremarkable, revealing normal arm swing, normal tone, and absence of tremor or abnormal movements. Abnormal Involuntary Movement Scale (AIMS) score was 0. He was able to move his tongue in and out, side to side, without objective abnormalities. He subjectively reported that it was more difficult than usual to move his tongue. His speech was subtly dysarthric. There was no evidence of tongue swelling. The patient was not having any difficulty breathing or swallowing.The presentation was consistent with a mild dystonic reaction. Diphenhydramine 25 mg was administered by mouth. Within 30 minutes of administration, the patient's symptoms resolved. His scheduled dosage of risperidone was not administered that evening. …