Which drug interactions are particularly important with simvastatin?

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Simvastatin is mainly cleared by the liver via the enzyme CYP3A4. When a drug that strongly inhibits CYP3A4 is taken at the same time, simvastatin levels in the blood can rise markedly. This increases the risk of statin-associated muscle injury, including myopathy and rare rhabdomyolysis. Because of this, the recommended approach is to avoid combining simvastatin with strong CYP3A4 inhibitors or to reduce the statin dose and monitor closely if the combination is unavoidable. Examples of strong inhibitors include certain azole antifungals (like ketoconazole and itraconazole), macrolide antibiotics (such as clarithromycin), HIV protease inhibitors, and similar agents. Grapefruit juice can raise statin levels as well, but the most important and consistently emphasized interaction for simvastatin is with strong CYP3A4 inhibitors. Fibrates with statins can increase myopathy risk too, so caution is warranted, but the primary mechanism that drives the need for dose adjustment or avoidance is CYP3A4 inhibition. Vitamin D supplementation does not increase statin potency.