The standard, cheapest and best-studied iron salt, supplying about 20% elemental iron — a 325 mg tablet delivers 65 mg of iron. It is the reference treatment for iron deficiency anaemia worldwide.
Iron is the clearest illustration of this catalogue's central point in both directions. In genuine iron deficiency — very common in menstruating women, in pregnancy, in vegetarians and after blood loss — iron supplementation reliably raises haemoglobin and resolves the fatigue, breathlessness and impaired exercise capacity that come with it. Taken without deficiency, it does nothing beneficial, causes constipation and nausea in a large minority, and is genuinely dangerous in undiagnosed haemochromatosis. Iron is also the leading cause of fatal poisoning in young children among household products, which is why US products carry a mandated warning. Nobody should take iron without knowing their ferritin.
Sold as
Tablet, Modified-release tablet, Oral solution, Drops for infants, Component of multivitamins
Also known as
Ferrous sulphate, Iron sulfate, Elemental iron 65 mg
Regulation: The Tolerable Upper Intake Level is 45 mg/day of elemental iron for adults, set on gastrointestinal effects — a figure that ordinary treatment doses for anaemia deliberately exceed under medical direction. US law requires iron-containing supplements to carry a warning about accidental overdose in children, following a series of child deaths that led to unit-dose packaging rules.
What the research used
The amounts, forms and durations published trials of Iron (ferrous sulfate) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
8–18 mg
Strong
8 mg/day for men and postmenopausal women, 18 mg/day for menstruating women and 27 mg/day in pregnancy. Vegetarians are advised to aim considerably higher because non-heme iron is absorbed less efficiently.
Studied for Tolerable Upper Intake Level, adults
45 mg
Strong
Based on gastrointestinal effects rather than systemic toxicity. Treatment doses for anaemia commonly exceed it, which is appropriate under medical supervision and not otherwise.
Studied for Iron deficiency anaemia treatment
60–120 mg
Strong
Elemental iron daily, or 60–120 mg on alternate days · 3 months, then reassessment
Studies of hepcidin dynamics found that a dose raises hepcidin for around 24 hours, blunting absorption of a second dose — so alternate-day single doses achieve better fractional absorption with fewer side effects than daily divided dosing. This has changed practice in several countries.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
⏳On an empty stomach
Strong
Iron is absorbed considerably better fasted — food can reduce absorption by roughly half — but causes more nausea that way. Taking it an hour before food or two hours after is the standard compromise, and this trade-off has been measured directly.
Timing was tested. A trial compared this window against another and reported a difference.
🕐Any time
Strong
Calcium, tea and coffee polyphenols, phytate and antacids all substantially reduce iron absorption; vitamin C increases it. Separating iron from dairy, tea, coffee and calcium supplements by a couple of hours is well supported by absorption studies.
Timing was tested. A trial compared this window against another and reported a difference.
🌅Morning
Moderate
Hepcidin, the hormone that blocks iron absorption, is lowest in the morning, and alternate-morning dosing produced the highest fractional absorption in controlled studies. One of the few genuinely tested timing findings in this entire catalogue.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Strong
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
In iron deficiency anaemia, oral iron raises haemoglobin and ferritin and resolves the associated fatigue and reduced exercise capacity — one of the most consistently reproduced findings in clinical nutrition, across decades of trials. In people with normal iron stores, supplementation does not improve energy, performance or anything else, and carries real downsides.
What may be going on: Iron is the oxygen-binding centre of haemoglobin and myoglobin and a cofactor in mitochondrial electron transport. Deficiency limits oxygen delivery and cellular respiration.
Strong evidence. Consistent findings across multiple good-quality human studies.
Iron supplementation reduces fatigue in women with low ferritin even without anaemia, in several randomised trials. The effect is modest, and in women with normal iron stores there is no benefit.
Moderate evidence. Several human studies point the same way, with real caveats.
Iron supplementation in pregnancy reduces maternal anaemia and iron deficiency at term. Whether all pregnant women should take it routinely, or only those with demonstrated deficiency, differs between national guidelines.
Moderate evidence. Several human studies point the same way, with real caveats.
In iron-deficient athletes, correcting the deficiency improves endurance measures. Supplementing iron-replete athletes has not been shown to improve performance and risks iron overload over time.
Limited evidence. Early human work, or mostly lab and animal research.
Systematic review · Journal of the American Medical Directors Association · 2018 · PMID 28711425
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
A leading cause of fatal poisoning in young children
Iron tablets look like sweets and a small number can kill a toddler — accidental iron overdose was for years the leading cause of poisoning death in children under six in the US, which is why unit-dose packaging and warning labels were mandated. Keep iron-containing products, including prenatal and gummy multivitamins, out of reach and sight, and contact a poison centre immediately after any suspected ingestion rather than waiting for symptoms.
Avoid
Haemochromatosis and iron overload
Hereditary haemochromatosis is common in people of northern European descent and causes progressive iron accumulation damaging the liver, heart, pancreas and joints. Iron supplements accelerate that damage. Anyone with a family history, unexplained liver enzyme elevation, joint pain or diabetes should have iron studies before supplementing. The same applies to people who receive regular transfusions.
Interaction
Interferes with several medicines, and several reduce its absorption
Iron substantially reduces absorption of levothyroxine, tetracyclines, fluoroquinolones, bisphosphonates, levodopa and methyldopa — separation by at least two to four hours is standard. In the other direction, proton pump inhibitors, H2 blockers and antacids reduce iron absorption by raising gastric pH.
Worth knowing
Deficiency needs a cause, not just a supplement
Iron deficiency in men and postmenopausal women warrants investigation for gastrointestinal blood loss, including bowel cancer, and coeliac disease is a common overlooked cause at any age. Correcting the number without finding the reason can delay a serious diagnosis.
Too much
Constipation, nausea and black stools
Gastrointestinal effects affect a large minority: constipation, nausea, cramping, and harmless black stools. Alternate-day dosing, lower doses or a different iron salt often help. Black stools from iron can mask the appearance of gastrointestinal bleeding, which matters if bleeding is the reason for the deficiency.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.