Dietary intake of methylxanthines, caffeine in particular, decreases theophylline metabolism by acting as a substrate for metabolizing enzymes. Smoking of cigarettes has a profound effect on theophylline metabolism. There is a dose-related increase in theophylline clearance, with heavy smokers metabolizing theophylline twice as fast as nonsmokers. Drug interactions are of major clinical importance Cimetidine, macrolide antibiotics, and quinolones significantly decrease theophylline metabolism. Phenytoin, phenobarbital, and rifampin significantly increase theophylline metabolism.
Most studies report slower clearance in the elderly.
Before calculating an initial dose or adjusting the maintenance dose the program must know the dosage form, whether the patient is in acute congestive failure and whether any interacting drugs are being concurrently administered. The program calculates an ideal loading dose and maintenance dose based on lean body weight and the presence or absence of various factors which affect theophylline metabolism. The user then chooses a practical dose and the program calculates an estimated steady-state serum level. Dosage adjustment based on serum levels For IV dosage adjustment, the program utilizes either the linear method or the Chiou method. For oral dosage adjustment, only the linear method may be used.