L-Selenomethionine is widely used as a nutritional supplement due to its role as a bioavailable source of selenium, an essential trace element. It supports antioxidant defense systems in the body by being incorporated into selenoproteins such as glutathione peroxidase, which help reduce oxidative stress. It is commonly added to multivitamin formulations and selenium-enriched foods to prevent selenium deficiency, which can affect thyroid function and immune health. In medical research, L-selenomethionine is studied for its potential in reducing the risk of certain cancers, particularly prostate and colorectal cancers, due to its antioxidant properties and ability to support DNA repair mechanisms. It is also used in cell culture media to support the growth of mammalian cells, especially in the production of selenoproteins for biochemical studies. Additionally, it has been explored in radiation therapy as a radioprotective agent, helping to protect healthy tissues due to its free radical scavenging activity.