Oral terbutaline alone and in combination with theophylline: dose, plasma concentration, and effect in long-term treatment of bronchial asthma.

Oral terbutaline, in gradually increasing doses from 2.5 to 10 mg three times daily (t.i.d.) was administered to 12 patients with chronic bronchial asthma. There was a linear relationship between dose and steady-state plasma concentrations in individual patients, but the plasma levels varied fourfold between patients taking similar doses. The need for other medication tended to decrease, and the symptom score, peak expiratory flow rate (PEFR) and forced expiratory volume in one second (FEV1) improved significantly during the treatment, with a roughly linear dose-effect relationship for the doses 2.5, 5 and 7.5 mg t.i.d. An increase in the dose from 7.5 to 10 mg did not improve PEFR and FEV1 any further. Side-effects were generally few and mild. Oral theophylline 200 mg t.i.d. added to terbutaline 10 mg t.i.d. brought about a further improvement in pulmonary function without adding any troublesome side-effects. Our data indicate that there is little to gain by prescribing terbutaline doses higher than 7.5 mg t.i.d. Instead, theophylline might be added, since this combination seems to have a favourable therapeutic index.