Important: This page is for informational purposes only, based on published peer-reviewed research and official UK dietary guidelines (NHS, EFSA, SACN). It does not constitute medical advice. Always consult your GP or pharmacist before starting, stopping, or combining supplements.
Iron — Forms, Dosage & Interactions
Also known as: ferrous, ferric, iron bisglycinate, ferrous sulfate, ferrous fumarate
Overview
Iron is an essential trace mineral and one of the most abundant elements in the human body, with an adult body containing approximately 3–4 grams in total. It is required for the synthesis of haemoglobin — the oxygen-carrying protein in red blood cells — as well as myoglobin, which stores oxygen within muscle tissue. Beyond oxygen transport, iron is integral to mitochondrial energy production, DNA synthesis, and the activity of numerous enzyme systems involved in immune function and neurotransmitter metabolism. Iron deficiency is the most prevalent nutritional deficiency worldwide and remains common in the UK, particularly among women of reproductive age, pregnant women, adolescents, and those following vegetarian or vegan diets. People supplement iron most commonly to correct confirmed deficiency, address fatigue associated with low ferritin levels (even in the absence of frank anaemia), or to support increased physiological demands during pregnancy. The evidence base for supplementation in clinically confirmed deficiency is considered strong, with consistent improvements observed in haemoglobin, ferritin, and fatigue markers. However, supplementation without established need is not recommended, as excess iron accumulation carries meaningful health risks. Individual responses to supplementation may vary considerably based on baseline iron status and dietary patterns.
UK Dosage Guidelines
| Guideline | Value | Source |
|---|---|---|
|
Reference Nutrient Intake (RNI)
The amount sufficient for most people |
8.7 mg (men), 14.8 mg (women 19-50) | NHS / SACN |
|
Tolerable Upper Level (UL)
Maximum daily intake unlikely to cause harm |
17 mg (supplemental, UK EVM) | EFSA / SACN |
Forms Comparison
Iron is available in several supplemental forms. Bioavailability and suitability vary.
| Form Name | Bioavailability | Notes |
|---|---|---|
| Iron Bisglycinate | high | Chelated, well-absorbed, gentlest on stomach |
| Ferrous Fumarate | moderate | Common prescription form, effective but more GI side effects |
| Ferrous Sulfate | moderate | Most prescribed, cheapest, most GI side effects |
When to Take Iron
Recommended Time
☀️ Morning — research suggests taking Iron in the morning
Additional Notes
Best absorbed on an empty stomach with Vitamin C. Avoid with tea, coffee, calcium, or zinc within 2 hours.
With or Without Food
Research suggests taking Iron on an empty stomach for optimal absorption.
Known Interactions
14 known interactions with other supplements.
Both iron and Vitamin B12 are essential for red blood cell production. Deficiency in either can cause anaemia, and research suggests addressing both simultaneously when levels are low.
Action: If blood tests indicate both iron and B12 are low, supplementing both may be more effective than addressing one alone. They can generally be taken at the same time.
Read full analysis →Vitamin C significantly enhances non-heme iron absorption. Studies indicate it can increase iron uptake by 2-3 times when taken together.
Action: Taking iron with a source of Vitamin C (supplement or citrus juice) is a well-established strategy to improve absorption.
Read full analysis →Research indicates that zinc and iron compete for absorption via the DMT1 transporter when taken simultaneously. Studies suggest this can reduce absorption of both minerals by 30-50%.
Action: Taking these at least 2 hours apart may help avoid the absorption competition. For example, iron in the morning and zinc with lunch or dinner.
Read full analysis →Calcium can significantly inhibit non-heme iron absorption. Studies show reductions of 40-60% when taken together at doses above 300mg calcium.
Action: Research supports separating calcium and iron supplements by at least 2 hours for optimal absorption of both.
Read full analysis →Iron and manganese compete for the same intestinal transporter (DMT1). Research suggests high iron intake can reduce manganese absorption and vice versa.
Action: If supplementing both, taking them at different meals may improve absorption of each.
Read full analysis →Research suggests curcumin may chelate iron, potentially reducing its absorption. This may be beneficial for those with iron overload but problematic for those with iron deficiency.
Action: Individuals with low iron status may benefit from separating iron supplements from turmeric/curcumin by a few hours.
Read full analysis →Research suggests inorganic iron (ferrous sulfate) may oxidise and reduce Vitamin E in the gut. This interaction is less relevant with chelated iron forms.
Action: If taking both, separating them by a few hours or using a chelated iron form (ferrous bisglycinate) may reduce the interaction.
Read full analysis →Copper and iron have a complex absorption relationship. Research suggests high-dose iron can reduce copper absorption, though copper is also needed for iron metabolism.
Action: At typical supplement doses, this interaction is rarely clinically significant. If taking high-dose iron, copper status may be worth monitoring.
Read full analysis →Iron supplements may create an unfavorable gut environment for certain probiotic strains. Research suggests unabsorbed iron in the colon can alter the gut microbiome.
Action: If taking both, separating iron from probiotics by 2 hours may reduce any negative interaction.
Read full analysis →Research suggests iron and chromium may compete for absorption when taken simultaneously, particularly at higher doses.
Action: Separating iron and chromium supplements by a few hours may optimise absorption of both.
Read full analysis →Iron deficiency can impair thyroid function and iodine utilisation. Research suggests that iron deficiency reduces the effectiveness of iodine supplementation for thyroid health.
Action: Addressing iron deficiency alongside iodine supplementation may improve thyroid outcomes. Both can generally be taken at the same time.
Read full analysis →Both iron and Vitamin A (retinol) are stored in the liver. Research suggests excessive supplementation of both may increase liver burden, particularly in individuals not deficient.
Action: Iron supplementation in men and postmenopausal women is generally only recommended when a deficiency is confirmed by blood tests. Routine iron supplementation without testing may carry risks.
Read full analysis →NAC's thiol group can chelate iron in the gut. Research suggests separating them for optimal absorption of both, though NAC may also have hepatoprotective benefits against iron-induced oxidative stress.
Action: Taking NAC on an empty stomach and iron with a meal (ideally with Vitamin C) at a different time may optimise absorption of both.
Read full analysis →Certain amino acids in collagen (particularly glycine) may form complexes with iron that affect absorption. Research on this specific interaction is limited.
Action: If taking both, separating them by an hour may be prudent, though the clinical significance at typical doses is uncertain.
Read full analysis →Top Iron Products on AIScored
Check interactions with your other supplements
Add Iron to our interactive Stack Analyzer and see how it works with everything else you take.
Add Iron to your stack →Related Ingredients
Frequently Asked Questions
Iron bisglycinate (chelated iron) is generally associated with fewer gastrointestinal side effects than ferrous sulfate or ferrous fumarate. A systematic review and meta-analysis by Tolkien et al. (2015, PLoS One) found that ferrous sulfate caused significantly higher rates of nausea, constipation, and abdominal discomfort compared to placebo. Chelated forms appear better tolerated at comparable doses, though individual responses may vary.
Iron is generally better absorbed on an empty stomach, but this can worsen gastrointestinal tolerability. Taking iron alongside vitamin C-rich foods or a small meal can improve tolerance without substantially reducing absorption. The NHS advises avoiding tea, coffee, dairy products, calcium supplements, and antacids close to the time of taking iron, as these can significantly inhibit absorption.
Research suggests that haemoglobin levels in iron-deficient individuals typically begin improving within two to four weeks of consistent supplementation. However, complete replenishment of iron stores — reflected by normalised serum ferritin — generally requires three to six months of continued supplementation. Individual responses may vary based on baseline deficiency severity, supplement form, dietary iron intake, and underlying absorption factors. Follow-up blood tests are advisable to monitor progress.