Sodium selenite is an inorganic substance that may be toxic to humans and to the environment. They may be genotoxic and may cause cancer. It is found in multivitamins.
One article (1) looked at sodium selenite and its risks and benefits in the treatment of cancer. There have been some studies indicating that sodium selenite is toxic to humans, while others indicate that it is a substance that acts to fight cancer.
The article noted that just because selenium is a natural substance, it doesn’t mean that it Is healthy for you. Unfortunately, statistics from the CDC indicate that more than half of the US population takes a multivitamin containing selenium each day. There is, however, a difference between synthetic selenium and natural sources of selenium. Natural selenium may help cancer, while synthetic selenium may cause cancer.
Source: 1.http://articles.mercola.com/sites/articles/archive/2011/08/22/is-your-multivitamin-toxic.aspx. Accessed 9/6/16.