Phenytoin for motion sickness: clinical evaluation.

Phenytoin has previously been shown to protect against motion sickness induced by Coriolis stimulation. The purpose of our series of investigations was to investigate further the efficacy of phenytoin for motion sickness prophylaxis and to gain insight into its mechanism of action. We tested participants with electronystagmography, off-vertical rotation, sea travel, and parabolic flight after they received phenytoin or placebo. Blood levels of at least 9 micrograms/mL were found to protect against motion sickness. Electronystagmography showed significant decreases in the gain of the vestibuloocular reflex in participants receiving phenytoin. Few side effects were seen with drug levels in the 9 to 15 micrograms/mL range. Phenytoin is an effective motion sickness countermeasure that may exert its effect through a combination of central nervous system and peripheral vestibular effects.