alkaloidal drug used in medicine as an antiasthmatic, coronary vasodilator, and diuretic. Theophylline is a xanthine alkaloid, a methylxanthine chemically related to caffeine and theobromine. Along with caffeine, it is an active constituent of tea (Camellia sinensis), but it is commercially produced in pharmaceutical manufacture by chemical synthesis.

Administered orally or rectally in the treatment of asthma, it facilitates breathing by relaxing the bronchioles in the lungs. It is administered by injection for the treatment of congestive heart failure to stimulate the heart and increase the total output of blood by the heart. Given orally, it acts as a diuretic by inhibiting reabsorption in the renal (kidney) tubules.