You should not take simvastatin if you are allergic to it, if you have liver disease, or if you are pregnant or breast-feeding.
In rare cases, simvastatin can cause a condition that results in the breakdown of skeletal muscle tissue, leading to kidney failure. This condition may be more likely to occur in older adults and in people who have kidney disease or poorly controlled hypothyroidism (underactive thyroid). You may also be more likely to develop this condition if you are of Chinese descent and you take high-dose simvastatin while also taking medication that contains niacin (Advicor, Niaspan, Niacor, Simcor, Slo-Niacin, and others).