Critical flicker frequency (CFF) is a neurophysiological variable which can be measured precisely and which shows little diurnal fluctuation. In a double blind cross-over study on six male volunteers, hydroxyzine hydrochloride 25 mg caused sedation that was recognised subjectively. It was also assessed quantitatively by changes in CFF in which the effect of hydroxyzine appeared after the second hour, persisted during the third and fourth hours, but had disappeared after the seventh hour.
Key words Hydroxyzine - vigilance test - sedation - critical flicker frequency