Home Ingredients Ferrous Fumarate
EnergyWomen's Health

Ferrous Fumarate

Evidence Level
Strong
4 Clinical Trials
4 Documented Benefits
4/5 Evidence Score

Ferrous fumarate is an iron salt that is 33 percent elemental iron by weight, compared with 20 percent for ferrous sulfate and 12 percent for ferrous gluconate. In a randomized trial in 120 pregnant women with low hemoglobin, it raised hemoglobin as well as ferrous sulfate, ferrous ascorbate and carbonyl iron, but had the highest rate of side effects (56.7 percent). In 176 iron-deficient blood donors, fatigue eased about as much on oral ferrous fumarate as on an iron infusion, though there was no placebo group. At low doses in pregnancy (20 to 60 mg of iron a day) digestive complaints did not rise with dose, while 80 mg a day brought more constipation. Vitamin C helps absorption, tea and coffee reduce it, and calcium may lower it. Iron is only useful for low iron confirmed by a blood test.

Studied Dose 20 to 80 mg of iron a day in pregnancy trials; 100 mg capsules over 8 to 12 weeks in blood donors; 200 mg/day in one pregnancy treatment trial. Doses above the 45 mg/day adult upper limit are for low iron confirmed by a blood test.
Active Compound Ferrous fumarate

Benefits

Raises hemoglobin in pregnancy as well as other iron salts

In a randomized trial of 120 pregnant women with low hemoglobin, 200 mg of iron a day as ferrous fumarate, ferrous sulfate, ferrous ascorbate or carbonyl iron worked equally well for hemoglobin and other blood measures, and all four were judged equally cost-effective. Side effects were most common on ferrous fumarate (56.7 percent).

Supported by Trial 1

Higher elemental iron content

Ferrous fumarate is 33 percent elemental iron by weight, compared with 20 percent for ferrous sulfate and 12 percent for ferrous gluconate, so less of the salt is needed for the same amount of iron. Supplement Facts panels list the elemental iron, so that is the number to compare.

Supported by Trial 1, Trial 2

Digestive tolerability compared with other iron salts

Ferrous fumarate is not reliably gentler than ferrous sulfate. In 404 pregnant women, 20 to 60 mg of iron a day as fumarate caused no dose-related rise in digestive complaints, but 80 mg brought more constipation. Comparing two trials, ferrous bisglycinate at 25 mg caused fewer complaints than fumarate at 40 mg or sulfate at 50 mg.

Supported by Trial 2, Trial 1

Less fatigue when iron stores are low

The placebo-controlled fatigue trials used ferrous sulfate, which supplies the same ferrous iron as fumarate. In 198 fatigued, non-anemic women with ferritin below 50 ug/L, 80 mg/day of elemental iron cut fatigue scores 47.7% vs 28.8% on placebo over 12 weeks. In 176 iron-deficient blood donors, fatigue improved on oral ferrous fumarate and IV iron alike, but with no placebo arm. This applies only to low iron confirmed by a blood test.

Supported by Trial 3, Trial 1

Mechanism of action

1

Ferrous (Fe²⁺) Form

Like ferrous sulfate and gluconate, ferrous fumarate supplies iron in the ferrous (Fe²⁺) form, which is absorbed through the DMT1 transporter without first being reduced.

2

Fumaric Acid Carrier

Fumaric acid, the carrier, is a normal intermediate of the citric acid cycle that cells use to make energy.

3

Standard Iron Absorption and Function

Once absorbed, iron is built into hemoglobin, myoglobin, cytochromes and iron-sulfur enzymes, the same as iron from any source. In iron-depleted women, taking galacto-oligosaccharide fiber with ferrous fumarate raised the share of iron absorbed.

Clinical trials

1
Four Oral Iron Salts in Pregnancy
PubMed

Prospective, randomized, open-label, blinded-endpoint trial comparing ferrous sulfate, ferrous fumarate, ferrous ascorbate and carbonyl iron at 200 mg of elemental iron a day. (Gamad et al. 2021, Journal of Obstetrics and Gynaecology Research)

120 pregnant women with iron-deficiency anemia, 30 per group.

All four salts were equally effective at raising hemoglobin and other blood measures, and equally cost-effective. Adverse effects were most common in the ferrous fumarate group (56.7 percent).

2
Low-Dose Iron in Pregnancy and Digestive Complaints
PubMed

Two randomized, double-blind studies: ferrous fumarate at 20, 40, 60 or 80 mg iron a day, and ferrous bisglycinate 25 mg or ferrous sulphate 50 mg a day, from weeks 15 to 19 of pregnancy to delivery. (Milman et al. 2024, Journal of Pregnancy)

482 healthy women with an uncomplicated single pregnancy (404 in the fumarate dose study, 78 in the bisglycinate and sulphate study).

Between 20 and 60 mg of iron a day as fumarate, digestive complaints did not rise with dose; 80 mg brought significantly more constipation and laxative use. Across the two studies, bisglycinate 25 mg caused fewer complaints than fumarate 40 mg or sulphate 50 mg, which were similar. No woman needed to stop or change her iron.

3
Oral Ferrous Fumarate or Iron Infusion in Blood Donors
PubMed

Pre-planned secondary analysis of the IronWoMan randomized trial: one 1 g infusion of ferric carboxymaltose or 100 capsules of ferrous fumarate (100 mg iron with 20 mg vitamin C each) over 8 to 12 weeks. (Macher et al. 2020, Nutrients)

176 iron-deficient whole-blood and platelet donors in Austria (138 women, 38 men) aged 18 to 65 with ferritin 30 ng/mL or below.

Fatigue, restless legs and sleep quality improved after treatment (p < 0.001), as did headaches, breathlessness, dizziness and palpitations, with no difference between oral and intravenous iron. There was no placebo group. The study was supported by Vifor Pharma and G.L. Pharma, maker of the oral capsules.

4
Oral Ferrous Fumarate or Iron Infusion in Children with IBD
PubMed

Multicentre randomized trial (POPEYE) of one intravenous dose of ferric carboxymaltose against 12 weeks of oral ferrous fumarate. (Bevers et al. 2023, Journal of Pediatrics)

64 children aged 8 to 18 with inflammatory bowel disease and anemia at 11 centres.

At 1 month, 6-minute walking distance improved more with the infusion (z-score +0.71 vs -0.11), but there was no difference at 3 or 6 months. Hemoglobin rose at the same rate in both groups.

Side effects and drug interactions

Common Potential side effects

Digestive side effects such as constipation and nausea; in one pregnancy trial, side effects were more common on ferrous fumarate (56.7 percent) than on three other iron salts.
In pregnant women, constipation rose at 80 mg of iron a day but not at 20 to 60 mg.
Dark or black stools, which become more frequent at higher doses.
Taking iron with food can reduce stomach upset.
The adult upper limit is 45 mg of iron a day, and supplements with 25 mg or more can reduce zinc absorption.
Accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Keep iron out of reach of children, and in case of overdose call a doctor or poison control center immediately.

Important Drug interactions

Levothyroxine: iron reduces its absorption; labels advise not taking levothyroxine within 4 hours of iron.
Quinolone and tetracycline antibiotics, methyldopa and levodopa: iron can reduce their absorption; follow each medicine's label on spacing.
Bisphosphonates such as alendronate: take them with plain water at least 30 minutes before any other food, drink or medicine, as their labels direct.
Calcium: may reduce iron absorption; take calcium and iron supplements at different times.
Proton pump inhibitors: by lowering stomach acid they can reduce iron absorption.
Tea and coffee: polyphenol-rich drinks strongly reduce absorption of iron from a meal.
Vitamin C: enhances absorption of nonheme iron.

Featured in these supplement guides

Restless Legs Syndrome→Vertigo & Dizziness→

More Energy Ingredients

Energy
Caffeine
Caffeine is a stimulant found naturally in coffee, tea, cocoa, guarana and kola nut, added…
Energy
Iron
Iron is an essential mineral needed to make hemoglobin, the protein in red blood cells tha…
Energy
Ferrous Sulfate
Ferrous sulfate is the reference oral iron salt, the form that most other iron supplements…
Energy
Vitamin B12
Vitamin B12 (cobalamin) is a water-soluble vitamin needed for red blood cell formation, DN…

Related articles from our blog

📝 Article
Iron Bisglycinate vs Ferrous Sulfate: Gentler, or Just Pricier?
Ask about iron supplements and someone will tell you to skip ferrous sulfate , the cheap pharmacy standby, and buy iron…
October 31, 2026
📝 Article
Taking Iron Every Other Day: Does It Absorb Better?
If you have ever been told to take iron for low iron, you have probably been told to take it every day, sometimes twice…
October 26, 2026

Frequently asked questions about Ferrous Fumarate

What is ferrous fumarate?

Ferrous fumarate is a common iron salt with a high elemental iron content (33 percent), used to raise low iron levels. It is well absorbed but, like ferrous sulfate, can cause digestive side effects.

How much ferrous fumarate should I take?

A typical tablet provides a substantial amount of elemental iron, so doses are set by need. In pregnant women taking ferrous fumarate, digestive complaints did not rise with dose between 20 and 60 mg of iron a day, while 80 mg brought more constipation. Only take iron for a confirmed deficiency, ideally under medical guidance.

Does ferrous fumarate cause side effects?

Like other iron salts, it can cause constipation, nausea, cramping, and dark stools. Taking it with a little food can help. In pregnant women, 80 mg of iron a day caused more constipation than 20 to 60 mg, and in a separate comparison ferrous bisglycinate at 25 mg caused fewer complaints than ferrous fumarate at 40 mg.

How can I absorb ferrous fumarate better?

Take it with vitamin C (like orange juice) and away from coffee and tea, which can sharply cut iron absorption, and from calcium, which may lower it. If it upsets your stomach, taking it with a little food can help.

What is Ferrous Fumarate used for?

Ferrous Fumarate is researched primarily for Energy and Women's Health. In a randomized trial of 120 pregnant women with low hemoglobin, 200 mg of iron a day as ferrous fumarate, ferrous sulfate, ferrous ascorbate or carbonyl iron worked equally well for hemoglobin and other blood measures, and all four were ju…

Does Ferrous Fumarate interact with any medications?

Possible interactions include: Levothyroxine: iron reduces its absorption; labels advise not taking levothyroxine within 4 hours of iron. Quinolone and tetracycline antibiotics, methyldopa and levodopa: iron can reduce their absorption; follow each medicine's label on spacing. If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Ferrous Fumarate?

NutraSmarts rates the evidence for Ferrous Fumarate as Strong (4 out of 5). It is backed by 4 study summaries and 8 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(8 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Gamad N, Saha PK, Sharma P, Suri V, Chakrabarti A, Saha L. A randomized controlled trial comparing the efficacy, tolerability, and cost of oral iron preparations in iron-deficiency anemia in pregnancy. J Obstet Gynaecol Res. 2021;47(11):3828-3841. doi: 10.1111/jog.14999.PubMedUsed to support: Prospective, randomized, open-label, blinded-endpoint trial in 120 pregnant women with iron-deficiency anaemia (30 per group). Ferrous sulfate, ferrous fumarate, ferrous ascorbate and carbonyl iron at 200 mg elemental iron a day were equally effective for haemoglobin and other blood measures and equally cost-effective. Adverse effects were most common on ferrous fumarate (56.7 percent).
  2. Milman NT, Bergholt T. Low-Dose Prophylactic Oral Iron Supplementation (Ferrous Fumarate, Ferrous Bisglycinate, and Ferrous Sulphate) in Pregnancy Is Not Associated With Clinically Significant Gastrointestinal Complaints: Results From Two Randomized Studies. J Pregnancy. 2024;2024:1716798. doi: 10.1155/2024/1716798.PubMedUsed to support: Two randomized double-blind studies in 482 healthy pregnant women. With ferrous fumarate at 20, 40, 60 or 80 mg iron a day, complaints did not rise with dose between 20 and 60 mg, but 80 mg brought significantly more constipation and laxative use. Comparing the studies, ferrous bisglycinate 25 mg caused fewer complaints than ferrous fumarate 40 mg or ferrous sulphate 50 mg, which were similar. Black stools rose with dose. No woman needed to change or stop her iron.
  3. Jeroense FMD, Michel L, Zeder C, Herter-Aeberli I, Zimmermann MB. Consumption of Galacto-Oligosaccharides Increases Iron Absorption from Ferrous Fumarate: A Stable Iron Isotope Study in Iron-Depleted Young Women. J Nutr. 2019;149(5):738-746. doi: 10.1093/jn/nxy327.PubMedUsed to support: Stable-isotope study in 34 iron-depleted young women given 14 mg iron as ferrous fumarate. Adding 15 g of galacto-oligosaccharides raised fractional iron absorption by 61 percent when taken with water and 28 percent with a meal; 4 weeks of daily GOS did not add to this effect.
  4. Bevers N, Van de Vijver E, Aliu A, Rezazadeh Ardabili A, Rosias P, Stapelbroek J, Bertrams Maartens IA, van de Feen C, Escher H, Oudshoorn A, Teklenburg S, Vande Velde S, Winkens B, Raijmakers M, Vreugdenhil A, Pierik MJ, van Rheenen PF. Ferric Carboxymaltose Versus Ferrous Fumarate in Anemic Children with Inflammatory Bowel Disease: The POPEYE Randomized Controlled Clinical Trial. J Pediatr. 2023;256:113-119.e4. doi: 10.1016/j.jpeds.2022.12.016.PubMedUsed to support: Randomized trial at 11 centres in 64 children aged 8 to 18 with inflammatory bowel disease and anaemia: one intravenous dose of ferric carboxymaltose or 12 weeks of oral ferrous fumarate. Six-minute walking distance improved more with the infusion at 1 month, but not at 3 or 6 months, and haemoglobin rose at the same rate in both groups.
  5. Vaucher P, Druais PL, Waldvogel S, Favrat B. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin: a randomized controlled trial. CMAJ. 2012;184(11):1247-54. doi: 10.1503/cmaj.110950.PubMedUsed to support: Multicentre, closed-label, observer-blinded randomized trial in 198 French women aged 18 to 53 with fatigue, ferritin below 50 ug/L and haemoglobin above 12.0 g/dL. Oral ferrous sulfate (80 mg elemental iron a day) for 12 weeks lowered fatigue scores by 47.7 percent against 28.8 percent on placebo (p = 0.02), with no effect on quality of life, depression or anxiety. Haemoglobin rose 0.32 g/dL and ferritin 11.4 ug/L more than on placebo. This trial used ferrous sulfate, not ferrous fumarate.
  6. Macher S, Herster C, Holter M, Moritz M, Matzhold EM, Stojakovic T, Pieber TR, Schlenke P, Drexler C, Amrein K. The Effect of Parenteral or Oral Iron Supplementation on Fatigue, Sleep, Quality of Life and Restless Legs Syndrome in Iron-Deficient Blood Donors: A Secondary Analysis of the IronWoMan RCT. Nutrients. 2020;12(5):1313. doi: 10.3390/nu12051313.PubMedUsed to support: Pre-planned secondary analysis of a randomized trial in 176 iron-deficient blood donors (ferritin 30 ng/mL or less): one 1 g infusion of ferric carboxymaltose or 100 oral ferrous fumarate capsules (100 mg iron with 20 mg vitamin C each) over 8 to 12 weeks. Fatigue, restless legs and sleep quality improved (p < 0.001), with no difference between oral and intravenous iron. There was no placebo arm; the study was supported by Vifor Pharma and G.L. Pharma, maker of the oral capsules.
  7. Hurrell RF, Reddy M, Cook JD. Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages. Br J Nutr. 1999;81(4):289-95..PubMedUsed to support: Radio-iron absorption study in adults eating a bread meal. Coffee, black tea, cocoa and several herb teas all reduced non-haem iron absorption in proportion to their polyphenol content; black tea cut absorption by 79 to 94 percent, and adding milk made little difference.
  8. NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. National Institutes of Health. 2025;Online fact sheet, updated September 4, 2025..SourceUsed to support: Not PubMed-indexed (official NIH fact sheet). Gives the iron RDAs (8 mg a day for men and for women over 50, 18 mg for women 19 to 50, 27 mg in pregnancy) and the adult Tolerable Upper Intake Level of 45 mg a day; notes that supplements with 25 mg or more can reduce zinc absorption, that ferrous fumarate is 33 percent, ferrous sulfate 20 percent and ferrous gluconate 12 percent elemental iron, and that the WHO estimates about half of the 1.62 billion cases of anemia worldwide are due to iron deficiency. Lists interactions with levodopa, levothyroxine (labels advise a 4-hour gap) and proton pump inhibitors, advises taking calcium and iron supplements at different times, and quotes the required US warning that accidental iron overdose is a leading cause of fatal poisoning in children under 6.