Drowsiness and motor responses to consecutive daily doses of promethazine and loratadine

Limited information is available regarding sedation and motor function following repeat dosing of antihistamines. This study examined how promethazine and loratadine affect day-time drowsiness, the commencement of voluntary movement, and involuntary movement when administered on consecutive days.

Ten healthy young subjects (24±5years) were recruited into a double-blind, placebo-controlled, three-way crossover study. Subjects ingested either promethazine, loratadine or a placebo, and ingested the same drug 24h later. Measures of drowsiness, simple reaction time (SRT), choice reaction time (CRT), and postural tremor were obtained pre-ingestion, 1h post-ingestion and 2h post-ingestion on each day.

Consecutive daily doses of promethazine and loratadine affected SRT and CRT, respectively, whereby reaction time deficits were less pronounced following the repeat dose. A reduced tremor response was also observed following consecutive daily dosing of promethazine, in contrast to loratadine which caused an increase in tremor amplitude with the consecutive daily dose.

Reaction time and tremor responses differed following the single dose compared to consecutive doses.

Sufferers of allergic rhinitis often require antihistamine dosing regimens that continue over multiple days. Future studies will benefit from examining drowsiness and movement responses following single doses as well as consecutive dosing.