See also How to Use section.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist. Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include: "blood thinners" (such as warfarin), cyclosporine, danazol, daptomycin, gemfibrozil.

Other medications can affect the removal of simvastatin from your body, which may affect how simvastatin works. Examples include certain azole antifungals (such as itraconazole, ketoconazole, posaconazole, voriconazole), cobicistat, colchicine, macrolide antibiotics (such as clarithromycin, erythromycin), nefazodone, HIV protease inhibitors (such as nelfinavir), ritonavir, telithromycin, among others.

Chinese people should not take niacin at doses used to lower cholesterol (1 gram or more) when taking this medication. Doing so may increase the risk of side effects (especially muscle problems). Talk with your doctor for details.

Do not take any red yeast rice products while you are taking simvastatin because some red yeast rice products may also contain a statin called lovastatin. Taking simvastatin and red yeast rice products together can increase your risk of serious muscle and liver problems.