Iron is the supplement most likely to be genuinely needed — and also the one most likely to cause harm when taken without reason. Both halves of that sentence matter.
Who is actually at risk
The NIH Office of Dietary Supplements lists the main at-risk groups in its consumer sheet on iron: teenage girls and women with heavy periods, pregnant women and teens, premature or low-birth-weight infants, frequent blood donors, and people with cancer, gastrointestinal disorders or heart failure. Deficiency also occurs in people who do not eat meat, poultry or seafood.
Test, do not guess
Tiredness is a terrible diagnostic. It has dozens of causes, and iron is only one of them. A ferritin test (plus a full blood count) tells you where you stand, and it matters because unlike vitamin C or B12, surplus iron is not simply flushed out — the body has no efficient way to excrete it. Some people also carry hemochromatosis, a genetic condition causing iron overload, for whom supplementing is actively harmful.
Heme vs non-heme, and absorption
Iron from meat (heme) absorbs far better than iron from plants and supplements (non-heme). Two practical levers: vitamin C markedly improves non-heme absorption, so pairing iron with citrus or a vitamin C tablet helps; while tea, coffee, calcium and dairy taken at the same time reduce it. Separate them by a couple of hours.
Managing the side effects
Constipation, nausea and stomach pain are common, particularly at high doses on an empty stomach. Two things help: gentler forms such as ferrous bisglycinate, and — perhaps counterintuitively — taking it every other day rather than daily. Alternate-day dosing has been found to absorb comparably well while causing fewer complaints.
Gentle Iron / Ferrous Bisglycinate (e.g. Solgar, Thorne)
- Bisglycinate is gentler on the stomach than ferrous sulfate
- Take with vitamin C; keep tea, coffee and calcium a couple of hours apart
- Every-other-day dosing is often better tolerated
Inexpensive
Check Price →Safety notes — take these seriously
High doses can cause stomach inflammation and ulcers, and interfere with zinc absorption. Very large amounts cause organ failure and can be fatal. Iron overdose is a leading cause of poisoning deaths in young children — keep supplements out of reach and in child-resistant packaging. Do not start iron because you feel tired; start it because a test and a clinician said so.
Not sure whether you need a targeted supplement or a general one? See do multivitamins work.