Asynchronies of diphenhydramine plasma-performance relationships

The relation between performance on driving-related tasks and plasma levels of diphenhydramine was studied in eight male volunteers over 24 hours following oral administration. DPH plasma concentrations rose to peak levels in 1.5-2.5 hours varying with dose, declining to a nearly constant level by 12-24 hours. For all behavioral measures, the shape of performance curves over time was similar to that of plasma, reaching maximum decrements in 1-4 hours. The relation between plasma levels and performance was asynchronous varying with behavioral measure and dose. The use of plasma DPH values to predict performance decrements is limited due to the bivalued nature of these relationships in time. Nonetheless, it would appear that if DPH is administered in the therapeutic dosage range at the intervals typically recommended for cold symptoms and allergies, it appears some aspects of human performance may be impaired.