A phosphodiesterase inhibitor and bronchodilator, theophylline (1,3-dimethylxanthine) is prescribed for respiratory ailments such as asthma and chronic obstructive pulmonary disease (COPD). This xanthine derivative, with structural and pharmacological links to theobromine and caffeine, works by relaxing the muscles in the lung’s air passages. It is often used alongside other medications to manage symptoms of asthma, bronchitis, emphysema, and other lung diseases.