simvastatin and amlodipine

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

Elevated simvastatin levels increase the risk of dose-dependent adverse effects, particularly myopathy and rhabdomyolysis. Symptoms may include unexplained muscle pain, tenderness, weakness, and dark urine. The risk is greater with higher simvastatin doses.

If the combination is necessary, limit the simvastatin dose to 20 mg daily, or use an alternative statin that is less dependent on CYP3A4, such as pravastatin or rosuvastatin. Monitor patients for signs of myopathy, and consider measuring creatine kinase if symptoms develop. Do not exceed the recommended dose.

This interaction is well established and documented in simvastatin prescribing information. It is based on pharmacokinetic studies showing increased simvastatin exposure when coadministered with amlodipine, and clinical case reports of myopathy. Regional guidelines recommend dose adjustment or avoidance.