The rate of elimination of theophylline varies widely from individual to individual depending on age, body weight, diet, history of smoking, presence of hepatic or cardiovascular dysfunction, pneumonia, chronic obstructive lung disease, pneumonia, and viral exanthems. As a result of the variability in theophylline clearance and a narrow toxic/therapeutic ratio of theophylline plasma concentrations, the use of plasma concentration monitoring can ensure adequate and safe dose schedules for most patients.