The effect of erythromycin on theophylline pharmacokinetics at steady state.

We compared steady-state theophylline pharmacokinetics in 13 healthy adults before and immediately after erythromycin therapy. All subjects received a five-day course of oral aminophylline 3 mg/kg every six hours prior to and during a five-day course of oral erythromycin stearate (1 g daily). Each subject acted as his own control. Multiple serum samples were collected over ten hours following the last dose of aminophylline during both the control and experimental phases of the study. Erythromycin did not significantly affect theophylline clearance (P greater than 0.70), elimination (P greater than 0.75), or volume of distribution (P greater than 0.30). We found no evidence of a pharmacokinetic interaction between theophylline and erythromycin at steady state. Worsening pulmonary function may be responsible for altered theophylline pharmacokinetics in patients coincidentally receiving erythromycin.