Increase in Risperidone Plasma Level With Lamotrigine

Because of these interesting reports, we decided to supplement the clozapine-risperidone combination of Ms. A, a 26-year-old inpatient who suffers from therapy-resistant schizophrenia with imperative auditory hallucinations, with lamotrigine. She had been taking clozapine, 550 mg/day, for 5 years and risperidone, 8 mg/day, for 4 weeks and had only responded partially. Her plasma levels of risperidone (55–70 ng/ml) and clozapine (800–1100 ng/ml) were stable. Her lamotrigine dose was titrated up to 250 mg/day in steps of 25 mg per week. After Ms. A had been taking 175 mg/day of lamotrigine for 5 days, her plasma level was 5 mg/liter, and her risperidone plasma level was 69 ng/ml. We further increased her dose of lamotrigine to 200 mg/day. Her risperidone level rose to 284 ng/ml, and in follow-up measures, it showed a value of 263 ng/ml. Her clozapine level rose to 1300 ng/ml. Ms. A did not have any intoxication symptoms. Because we did not assume any connection of increased plasma level of risperidone with lamotrigine, we heightened the dose to 225 mg/day. The next measurement of both plasma levels indicated an exorbitant increase of risperidone plasma level, up to 412 ng/ml. Ms. A complained of dizziness and tiredness. We quickly reduced the dose of risperidone to 2 mg/day and withdrew the drug 1 week later. An overdose of risperidone was unlikely since it was taken under supervision.