simvastatin and diltiazem

Diltiazem inhibits the cytochrome P450 enzyme CYP3A4, which is responsible for the metabolism of simvastatin. This inhibition reduces simvastatin clearance, leading to increased plasma concentrations of simvastatin and its active metabolites.

Elevated simvastatin concentrations increase the risk of dose-related adverse effects, particularly myopathy and rhabdomyolysis. Patients may present with muscle pain, tenderness, weakness, dark urine, and elevated creatine kinase levels. In severe cases, rhabdomyolysis can lead to acute kidney injury.

Avoid concomitant use if possible. If co-prescription is necessary, limit the simvastatin dose to a maximum of 10 mg daily, as recommended by regulatory guidelines. Monitor patients for signs of myopathy, and consider measuring creatine kinase at baseline and if symptoms develop. Advise patients to report unexplained muscle pain or weakness promptly.

This interaction is well established and documented in the official prescribing information for simvastatin. Numerous clinical reports and pharmacokinetic studies confirm the increased exposure and risk of muscle toxicity.