• The margin between therapeutic and toxic doses of theophylline is narrow. Most people require plasma concentrations between 10 and 20 mg/L for satisfactory bronchodilation, although a lower concentration may be effective.
• Once a maintenance dose has been reached, check serum theophylline concentration every 6 to 12 months, or if the person is experiencing adverse effects that might suggest toxicity [UKMI, 2002].
• Adverse effects, including nausea, vomiting, tremor, palpitations, and arrhythmias, can occur at plasma concentrations of 10–20 mg/L. The frequency and severity of adverse effects increase with concentrations greater than 20 mg/L.
• Serum levels of theophylline are increased (because of an increase in the half-life of theophylline) in people with heart failure or hepatic impairment, in elderly people, and by drugs that inhibit hepatic enzymes (e.g. cimetidine, ciprofloxacin, erythromycin, fluvoxamine, St John's Wort):
• If people whose disease is stable during theophylline therapy begin to take one of these drugs, a reduction of the theophylline dose is recommended.
• Serum levels of theophylline are decreased (because of a decrease in the half-life of theophylline) in people who smoke, in chronic alcohol misuse, and by drugs that induce hepatic enzymes (e.g. phenytoin, carbamazepine, rifampicin):
• If people whose disease is stable during theophylline therapy begin to take one of those drugs, the theophylline dose may need to be increased.
• If people whose disease is stable during theophylline therapy stop smoking, plasma levels of theophylline may increase, and a reduction in dose may be necessary.
• When prescribing theophylline, the brand should be specified on the prescription. Because of differences in bioavailability among brands, people should be maintained on the same brand of theophylline.

[Baxter, 2006; BNF 53, 2007]