Selenium can be toxic to animals, causing serious health issues, especially in horses.
78 citations,
December 2011 in “Clinical toxicology” Excessive selenium from a supplement caused toxicity but patients recovered with care.
12 citations,
June 2023 in “Journal of trace elements in medicine and biology” Eating too much selenium can cause bad breath, hair loss, and nail changes, with harmful effects starting at low daily doses.
30 citations,
January 2023 in “EFSA journal” Adults should not consume more than 255 micrograms of selenium per day to avoid risk of hair loss and other side effects.
The conclusion is that hair supplement ingredients may be unsafe and should be disclosed and proven safe before use.