The human metabolism of caffeine to theophylline.

Theophylline peak plasma concentration of over 170 ng/ml average was observed when 300 mg of caffeine, equivalent to 2-3 cups of coffee, was administered to humans. This peak level was not observed until at least 7 hr post administration. Accumulation of caffeine, with its subsequent metabolism to theophylline, in patients, who consume average quantities of caffeine-containing beverages relative to those patients who avoid such drinks could interfere with bioavailability studies in normal volunteers. However, it may account for only a small portion of the variable clinical effects associated with aminophylline administration.