The pink coloured pentahydrate Na2SeO3(H2O)5 is the most common water-soluble selenium compound.

[4] Because selenium is an essential element, sodium selenite is an ingredient in dietary supplements such as multi-vitamin/mineral products, but supplements that provide only selenium use L-selenomethionine or a selenium-enriched yeast.

According to one article, "not much was known about which selenium compounds to approve for use in animal feeds when the decisions were made back in the 1970s .. At the time the regulatory action was taken, only the inorganic selenium salts (sodium selenite and sodium selenate) were available at a cost permitting their use in animal feed.” [5] Sodium selenite has been proposed as an effective suicide agent.

As sodium selenite, the chronic toxic dose for human beings was described as about 2.4 to 3 milligrams of selenium per day.

[7] In 2000, the US Institute of Medicine set the adult Tolerable upper intake levels (UL) for selenium from all sources - food, drinking water and dietary supplements - at 400 μg/day.