Theophylline (1,3-dimethylxanthine) is a metabolite of caffeine and an alkaloid found in the cacao and to a lesser extent in tea plants. It is a phosphodiesterase inhibitor, an adenosine receptor antagonist, and a central nervous system stimulant. It was once used as a bronchodilator to treat asthma; however, it has been replaced by drugs with fewer side effects.