Urinary retention caused after fluoxetine-risperidone combination

A case of urinary retention emerging after fluoxetine (20 mg/day) addition to low risperidone doses (2 mg/day) is presented. Severe extrapyramidal side-effects (EPS) also occurred after fluoxetine-risperidone combination. Several possibilities, based on the pharmacodynamic and pharmacokinetic properties of risperidone and fluoxetine, which merit consideration in an attempt to explain our patient's side-effects, are discussed. Extrapyramidal side-effects can be due to an increase of the plasma concentration of risperidone and/or the intrinsic propensity of fluoxetine to produce EPS. Urinary retention may be the consequence of a central serotoninergic mechanism in, or without, combination central D2 blockade.