If you are currently being treated with any of the following medications, you should not use selenium supplements without first talking to your healthcare provider.

Cisplatin, Doxorubicin, and Bleomycin
Selenium may reduce toxic side effects associated with cisplatin and doxorubicin, two forms of chemotherapy used to treat cancer. On the other hand, a test tube study suggested that selenium may inhibit the anti-cancer effects of bleomycin.

Cholesterol-lowering Medications 
Researchers recently discovered an unexpected adverse interaction between antioxidant supplements and a popular combination of cholesterol-lowering medications known as simvastatin and niacin -- this interaction may have important implications for patients with heart disease. Together, simvastatin and niacin have been shown to lower LDL ("bad") cholesterol and raise HDL ("good") cholesterol in people with heart disease. When taken with antioxidants (including selenium), however, these medications may not be as effective in raising HDL cholesterol.