Oral xanthine derivatives (theophylline and doxofylline) for patients with stable chronic obstructive pulmonary disease (COPD)

To compare the efficacy and safety of adding oral theophylline or doxofylline to usual treatment (long-acting beta2-agonists, antimuscarinics, inhaled corticosteroids) versus providing usual treatment alone for patients with stable COPD.

This is a protocol.