simvastatin and erythromycin

Erythromycin is a strong inhibitor of CYP3A4, the cytochrome P450 enzyme responsible for the metabolism of simvastatin. Inhibition of CYP3A4 by erythromycin markedly increases the plasma concentrations of simvastatin and its active metabolites, leading to exaggerated systemic exposure and an increased risk of skeletal muscle toxicity.

Elevated simvastatin concentrations increase the risk of dose-dependent myopathy, which can progress to rhabdomyolysis, a potentially life-threatening condition associated with muscle breakdown, myoglobinuria, acute renal failure, and death. Symptoms may include muscle pain, tenderness, weakness, and dark urine.

Co-administration is contraindicated. If erythromycin is required, alternative lipid-lowering therapy should be considered, such as pravastatin or rosuvastatin, which are less dependent on CYP3A4 metabolism. If temporary treatment with erythromycin is unavoidable, simvastatin should be withheld during the course of antibiotic therapy.

This interaction is well-established and documented in the simvastatin prescribing information, which lists strong CYP3A4 inhibitors, including erythromycin, as contraindicated. Numerous case reports and clinical studies have confirmed the risk of rhabdomyolysis when simvastatin is combined with erythromycin.