Pharmacokinetic interaction between theophylline and erythromycin.

1 The pharmacokinetics of a single intravenous dose of theophylline alone and during the fifth day of treatment with 500 mg erythromycin every 8 h was studied in six healthy subjects. 2 At the same time the pharmacokinetics of erythromycin at steady-state on the fourth day of the treatment (alone) and the fifth day (during the theophylline co-administration) were also studied. 3 Mean +/- s.d. theophylline clearance was decreased from 62 +/- 15.4 to 53 +/- 10.3 ml min-1 (P less than 0.05) and elimination half-life rose from 7.1 +/- 1.9 to 7.7 +/- 2 h (P less than 0.05) when erythromycin was co-administered. 4 Mean +/- s.d. erythromycin area under the curves (0-8 h) and (0-oc) were reduced from 6.09 +/- 3.2 to 3.8 +/- 2.5 micrograms ml-1 h and 7.2 +/- 3.6 to 5.0 +/- 2.9 micrograms ml-1 h (P less than 0.05) in the presence of theophylline. Mean steady state and maximum steady state concentrations were also reduced from 0.75 +/- 0.4 to 0.47 +/- 0.3 microgram ml-1 (P less than 0.05) and 1.45 +/- 0.87 to 0.85 +/- 0.51 microgram ml-1 (P less than 0.05) respectively. 5 The potential clinical implications of this indication should be considered.