Increased AUC w/ HIV-PIs, saquinavir, ritonavir. Increased blood conc & efficacy w/ ritonavir. Increased hypotensive effects w/ other hypotensive drugs, fentanyl, cimetidine, tandospirone citrate. Increased hypotensive & negative inotropic effect w/ β-blockers eg, propranolol. Decreased renal clearance & increase blood conc of digoxin. May inhibit the enzyme P450 w/ immunosuppresants eg, ciclosporin, tacrolimus hydrate. May accelerate the metabolism of Ca antagonists as rifampicin induces metabolism of drug enzyme in the liver. Hypercalcemia may occur w/ dantrolene Na. Increased effect of muscle relaxants eg, pancuronium Br, Vecuronium Br. Nitroglycerin.