Increased exposure w/ concomitant use of strong or moderate CYP3A4 inhibitors [protease inhibitors, azole antifungals, macrolides (eg, erythromycin or clarithromycin), verapamil or diltiazem]. May decrease plasma conc w/ CYP3A4 inducers (rifampicin, Hypericum perforatum
). Bioavailability may be increased w/ grapefruit juice resulting in increased BP lowering effects. Potentially, lethal ventricular fibrillation & CV collapse associated w/ hyperkalemia after co-administration w/ dantrolene IV. Additive BP-lowering effects w/ other antihypertensives. Limit simvastatin dose to 20 mg daily in concomitant use.