Large amounts of selenium are toxic; however, it is physiologically necessary for animals in extremely small amounts. Many other uncharacterized selenium-containing organic chemicals are also produced by a method similar to that of selenomethionine; some have recently been characterized but remain relatively unknown, such as S-seleno-methyl-glutathione and glutathione-S-selenoglutathione. Due to this, the European Union has questioned the safety and potential toxicity of this food supplement for humans, and it may not be used as an additive after 2002. 
[https://www.degruyter.com/document/doi/10.1351/pac200678010105/html G.N Schrauzer], who has written two papers about selenomethionine, claims it should be an essential amino acid, and that the product is completely safe. The European Food Safety Authority does allow the use of selenomethionine as a feed additive for animals. Because organic forms of selenium appear to be excreted from the body slower than inorganic forms, products enriched with organic selenium might detrimentally bioaccumulate in the body. Because selenium-enriched foods contain much more selenium than natural foods, selenium toxicity is a potential problem, and such foods must be treated with caution. The EU allows up to 300 micrograms of selenium per day, but one long-term study of selenium supplementation showed no evidence of toxicity at a dose as high as 800 micrograms per day.
An organic selenium-containing chemical found in selenium yeast has been shown to differ in bioavailability and metabolism compared with common inorganic forms of dietary selenium. Dietary supplementation using selenium yeast is ineffective in the production of antioxidants in bovine milk compared to inorganic selenium (sodium selenate). One study examined if increased selenium in the diet of mutant mice (via a selenium yeast product) caused a higher production of selenium-containing enzymes which have an antioxidant effect. The effect was modest.
Selenium supplementation in yeast form has been shown to increase pig selenium-containing antioxidant enzymes, broiler growth and meat quality, the shelf life of turkey and rooster semen, and possibly cattle fertility.
Selenium supplementation in animal feeds may be profitable for agribusinesses. It may be possible to market selenium-fortified foods to consumers as functional foods, such as selenium-enriched eggs, meat, or milk.