Selenium

Won't reliably prevent or treat peripheral neuropathy on its own. No proven benefit for glioblastoma survival when combined with other supplements. No evidence from these studies that it prevents cancer, boosts immunity, or improves heart health. The Alzheimer's nanoparticle research is entirely preclinical — don't expect brain benefits from a supplement capsule.

• Selenium has a very narrow safe range — the gap between a beneficial dose and a toxic dose is small. High doses can cause selenosis (hair loss, nail brittleness, nerve damage).
• High-dose IV selenium (2000 mcg) used in cancer trials is far above typical supplement doses and should never be self-administered.
• One large population study found higher blood selenium was positively associated with a cardiovascular risk marker (AIP), partially mediated by inflammation — suggesting more is not always better.
• Most exciting uses (Alzheimer's nanoparticles, cancer treatment) are preclinical animal or lab studies only — not proven in humans.
• Selenium in shampoo (selenium disulfide) is a topical antifungal, not a nutritional supplement — these are completely different applications.