The mechanism of action of theophylline in the treatment of asthma is not yet fully understood. Theophylline appears to be beneficial in some patients with steroid-dependent or nocturnal asthma. It is recommended as an alternative or additional treatment for patients with chronic mild, moderate or severe disease that does not respond to first- or second-line therapy with beta 2 agonists and anti-inflammatory medications such as cromolyn sodium, nedocromil (in children 12 years of age or older) or inhaled corticosteroids. However, theophylline therapy may cause bothersome side effects, including gastrointestinal distress, anxiety, insomnia and headache. These side effects can be minimized by beginning therapy with a low dosage and increasing the dosage slowly, until a therapeutic blood level is reached. Toxicity is more likely if the blood level exceeds 20 micrograms per mL. Prolonged fever puts patients at especially high risk for toxicity. Appropriate patient selection, careful dosing and regular monitoring are crucial elements of safe theophylline therapy.