BackgroundAntihistamines (histamine H1receptor antagonists) are effective and convenient drugs for the treatment of allergic conjunctivitis. Because of the short duration of action generally observed for drugs administered topically to the eye, the oral route is often preferred. However, the presence of a selective barrier between blood and ocular tissues, the so-called blood-ocular barrier, does not allowa prioriassessment of the most suitable dosage for ocular therapy.ObjectiveTo investigate the tear concentrations of cetirizine, a second-generation antihistamine, over time following oral administration, and to study the relationship between plasma and tear fluid concentrations.Design and participantsPharmacokinetic study of a single oral dose of cetirizine 10mg in 40 patients treated for allergic conjunctivitis.MethodsPatients received a single oral dose of cetirizine. Samples of blood and tear fluid were taken according to predefined sampling schedules and the concentrations of cetirizine were determined by a new high performance liquid chromatography method.ResultsConcentration-time profiles for cetirizine in serum and tear fluid were similar, although the mean maximum concentration in tear fluid was reached later than in serum (90 and 30 min, respectively). However, at 60 and 120 min the cetirizine concentration in tear fluid was 98 and 92% of the mean maximum concentration, respectively, showing a plateau region and indicating that the disposition rate for the tear fluid compartment was very similar to that for the blood compartment.ConclusionOral administration of cetirizine yields therapeutically effective concentrations of the drug at the anterior surface of the eye.