Recurrent sudden movements resembling startle responses accompany duloxetine treatment

Duloxetine is a widely used selective serotonin and norepinephrine reuptake inhibitor. Known motor side effects include tremor, muscle cramps and symptoms of serotonin syndrome like overactive reflexes and restlessness. Behaviours consistent with increased motor reactivity, such as increased startle response to noise and decreased habituation of locomotor activity, were observed in rat pups after their mothers had been orally exposured to 30mg/kg/day of duloxetine throughout gestation and lactation. To all our knowledge, motor side effects of this kind have not been described yet in humans treated with duloxetine. Here we report two cases of depressed patients developing identical movement disturbances soon after their duloxetine dosage had been increased. Both, a 70 year old woman and a 31 year old man, experienced annoying jerks simultaneously involving the whole body. The involuntary movements resembled startle responses, except that no trigger preceded them. They reoccurred in irregular intervals preferably during rest periods. Consciousness, body balance and recall were not disturbed. Serum concentrations and EEG recordings were within normal range. When reducing the dosage, the movements decreased in frequency and then completely disappeared. In the second case the patient was able to document the incidents with an actigraph watch showing that their course paralleled the descending serum concentrations.