Metabolism of theophylline to caffeine in adults.

Caffeine and its major metabolite, paraxanthine, were observed in plasma following oral administration of theophylline in a multiple dose study. At steady state, plasma caffeine concentrations varied from 0.21 to 0.75 mg/L at plasma theophylline concentrations of 8.1 to 21.5 mg/L in four healthy subjects. About 6% of the theophylline dose was converted to caffeine, which is further metabolized to a series of degradation products, which can be measured after caffeine administration, accounted for the incomplete urinary recovery usually observed after theophylline administration.