Theophylline-d6 is intended for use as an internal standard for the quantification of theophylline by GC- or LC-MS. Theophylline is an inhibitor of phosphodiesterase. It is also an adenosine A1 and A2 receptor antagonist. Theophylline induces relaxation of isolated cat bronchial smooth muscle segments precontracted with acetylcholine. It inhibits ovalbumin-induced increases in bronchoalveolar lavage fluid (BALF) eosinophil infiltration in an ovalbumin-sensitized mouse model of allergic asthma. Formulations containing theophylline have been used in the treatment of asthma and chronic obstructive pulmonary disease (COPD).