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Selenium
What Selenium is, what the NIH ODS fact sheet says the evidence supports, typical doses, timing, and the cautions that come with it.
Supplements are not medicines and this page is not medical advice. Doses that help one person can harm another — with medicines, kidney or liver conditions, pregnancy, or before surgery, talk to a clinician before starting anything. More is not better.
What it is
A mineral that is part of antioxidant enzymes and thyroid function.
What the evidence supports
ODS: treats deficiency; extra selenium has not been shown to prevent disease in selenium-sufficient people.
Dose and timing
RDA: 55 mcg per day. UL 400 mcg. Any time.
Cautions
Toxic in excess; the gap between enough and too much is narrower than most minerals.
Interactions are the hidden risk: supplements can change how medicines work and vice versa. Give your clinician the full list before anything else.
Source: NIH Office of Dietary Supplements — Selenium fact sheet · checked 2026-09-03
Frequently asked questions
What does Selenium do?
A mineral that is part of antioxidant enzymes and thyroid function.
Does Selenium work?
ODS: treats deficiency; extra selenium has not been shown to prevent disease in selenium-sufficient people.
How much selenium should I take?
RDA: 55 mcg per day. UL 400 mcg. Any time.
Is Selenium safe?
Toxic in excess; the gap between enough and too much is narrower than most minerals. And the standing rule: tell your clinician everything you take — supplements interact with medicines.
Creatine
Studied doses: 3-5 g per day; loading protocols use about 20 g/day in 4 doses for 5-7 days.
Magnesium
RDA: men 400-420 mg, women 310-320 mg per day.
Vitamin D
RDA: 600 IU (15 mcg) to age 70, then 800 IU (20 mcg). UL 4000 IU.
Omega-3s (EPA/DHA)
No RDA for EPA/DHA; ALA: men 1.6 g, women 1.1 g per day. Many products supply 250-500 mg combined EPA+DHA per day.
Melatonin
Studied doses commonly 0.5-5 mg, taken 30-60 minutes before bed.
Iron
RDA: men 8 mg, women 19-50 18 mg, pregnancy 27 mg per day.
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