Altered clearance w/ CYP450 inhibitors or inducers. Weakened efficacy by rifampicin; anti-epileptic drugs eg, phenytoin, carbamazepine, phenobarb. Increased plasma conc w/ macrolides eg, erythromycin; anti-HIV PIs eg, ritonavir; azole anti-mycotics eg, ketoconazole; fluoxetine, nefazodone, quinupristin/dalfopristin, valproic acid, cimetidine, cisapride; grapefruit juice. Increased BP-lowering effect of diuretics, β-blockers, ACE inhibitors, angiotensin II receptor antagonists, other Ca antagonists, α-adrenergic blocking agents, PDE5 inhibitors, α-methyldopa. Increased digoxin, quinidine plasma conc. Monitor tacrolimus conc. Falsely increased urinary vanillylmandelic acid spectrophotometric values.