The effect of single does of rifampicin on the pharmacokinetics of oral nifedipine.

The pharmacokinetic interaction of oral rifampicin (1200 mg) and oral nifedipine (10 mg capsules), given as single doses, was investigated in six healthy volunteers (mean age 28.5 +/- 6.3 years and mean weight 67.0 +/- 4/45 kg). The plasma concentrations of nifedipine was monitored using a HPLC technique, 8 h after pre-treatment, with rifampicin. The mean relative bioavailability of nifedipine following pre-treatment with rifampicin 1200 mg was 35.8% (P < 0.0001). The mean elimination half life (t1/2) of nifedipine decreased from 2.62 to 1.03 h (P < 0.0001); and, the total clearance (ClT) increased from 17.33 to 50.17 ml min-1 kg-1 (P < 0.0001). There were no significant differences in Vd and Tmax. The study suggests that the effect of induction by rifampicin decreases the bioavailability of nifedipine by either increasing the first pass effect or decreasing its oral absorption. The induction also increases the clearance of nifedipine.