Despite the widespread treatment of motion sickness symptoms using drugs and the involvement of the vestibular system in motion sickness, little is known about the effects of anti-motion sickness drugs on vestibular perception. In particular, the impact of oral promethazine, widely used for treating motion sickness, on vestibular perceptual thresholds has not previously been quantified. We examined whether promethazine (25 mg) alters vestibular perceptual thresholds in a counterbalanced, double-blind, within-subject study. Thresholds were determined using a direction recognition task (left vs. right) for whole-body yaw rotation, y-translation (interaural), and roll tilt passive, self-motions. Roll tilt thresholds were 31 % higher after ingestion of promethazine (P = 0.005). There were no statistically significant changes in yaw rotation and y-translation thresholds. This worsening of precision could have functional implications, e.g., during driving, bicycling, and piloting tasks. Differing results from some past studies of promethazine on the vestibulo-ocular reflex emphasize the need to study motion perception in addition to motor responses.