ABC of monitoring drug therapy. Theophylline.

It is not possible to establish a single dosage regimen for theophylline or its derivatives (for example, aminophylline) that will suit all patients because the metabolism of theophylline varies greatly from person to person. This variation is reflected in its elimination half life, which varies from four hours in healthy adult smokers to about 25 hours in patients with hepatic cirrhosis. Several of the factors responsible for the variation (age, smoking habits, body weight, diet, concomitant illness, and drug interactions) can be considered when estimating the most appropriate dosage regimen. But the therapeutic:toxic ratio for theophylline is very small, and only by measuring the plasma theophylline concentration can the dosage be tailored for individual patients. In this article we apply to theophylline the criteria that must be fulfilled in part or in full before the measurement of its plasma concentration can be considered worth while.