Iron Fumarate

Well-tolerated iron form with good bioavailability and reduced digestive side effects compared to iron sulfate

Scientific nameFerrous Fumarate FormulaC4H2FeO4 Molecular weight169.9 g/mol CAS141-01-5

25 absorption score

8 safety / 10

18 USD per month

32 elemental iron %

6 documented interactions

Check it against something

Evidence level A

Iron Fumarate with:

94 out of 100

Works better together Compatible. Timing differs, so follow each supplement's own window.

Check it against your whole stack →

What it does

mechanism and evidence for each claim

Superior digestive tolerance

Significantly less nausea and stomach upset than iron sulfate

Fumarate form causes less gastric irritation than sulfate Evidence A-grade Immediate

Good bioavailability and absorption

Well-absorbed with effective iron delivery to tissues

Fumaric acid enhances iron solubility and absorption Evidence A-grade 1-2 weeks

Effective anemia treatment

Proven efficacy for treating iron deficiency anemia

Provides readily bioavailable iron for hemoglobin synthesis Evidence A-grade 4-12 weeks

Reduced constipation risk

Less likely to cause constipation compared to sulfate forms

Better absorption reduces unabsorbed iron in digestive tract Evidence B-grade Immediate

Better long-term compliance

Improved tolerance leads to better supplementation adherence

Fewer side effects result in consistent daily use Evidence A-grade Ongoing

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Same goal, different mechanism — check any of them against this one

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Or a different form of the same thing

the comparison people search most

Iron Fumarate vs Iron Sulfate absorption 25 against 20 · $18 against $8 a month 20% Iron Fumarate vs Iron Bisglycinate absorption 25 against 90 · $18 against $30 a month 90% Iron Fumarate vs Heme Iron absorption 25 against 85 · $18 against $120 a month 85%

All Iron forms ranked by absorption →

Every pair we checked

12 combinations scored by the engine

Iron Fumarate + Turmeric May reduce iron absorption Needs attention 61

Iron Fumarate + Caffeine No documented interaction either way No known interaction 70

Iron Fumarate + Creatine Monohydrate No documented interaction either way No known interaction 70

Iron Fumarate + Calcium Carbonate No documented interaction either way No known interaction 70

Iron Fumarate + Whey Protein No documented interaction either way No known interaction 70

Iron Fumarate + Lactase No documented interaction either way No known interaction 70

Iron Fumarate + Bovine Collagen No documented interaction either way No known interaction 70

Iron Fumarate + DHA No documented interaction either way No known interaction 70

Iron Fumarate + Fish Oil (EPA/DHA) No documented interaction either way No known interaction 70

Iron Fumarate + Vitamin D3 Dramatically enhances iron absorption Works better together 82

Iron Fumarate + Copper Bisglycinate High iron may compete with copper absorption Needs attention 82

Iron Fumarate + Folate (5-MTHF) Dramatically enhances iron absorption Works better together 94

Dosage

With food for comfort

Daily65-130 mg

Maintenance30-65 mg elemental iron daily

Therapeutic100-200 mg elemental iron daily

Gentle supplementation30-100 mg elemental iron daily

Elemental ironContains ~33% elemental iron

Upper limit 45 mg NIH ODS

Before you take it

Pregnancycommonly recommended

VeganYes

When to expect it

First Effects1-2 weeks for energy and symptom improvements

Peak Benefits6-12 weeks for complete iron deficiency correction

Deficiency DevelopmentMonths to years depending on iron status

Toxicity OnsetRare with recommended doses

How to measure it

$50-150

MarkerHemoglobin, hematocrit, serum ferritin, transferrin saturation

OptimalHemoglobin: 12-15.5 g/dL (women), 13.5-17.5 g/dL (men), Ferritin: 15-200 ng/mL

Deficient belowHemoglobin: <12 g/dL (women), <13.5 g/dL (men), Ferritin: <15 ng/mL

FrequencyEvery 4-8 weeks during treatment, then every 3-6 months

What people actually reported to the FDA

CAERS, 340 reports naming it as the suspect

Most reported effects

Vomiting 33

Dyspnoea 31

Diarrhoea 28

Nausea 24

Hypersensitivity 23

Named as the suspect 340 reports of 1,000+ that mention it at all

Serious outcome 84% hospitalisation, emergency room or worse

Other Serious or Important Medical Event228

Hospitalization85

Visited Emergency Room65

Reports are voluntary and unverified — they record what someone experienced while taking something, not what the compound caused. Counts skew towards products people take a lot of, and a serious outcome is more likely to be reported than a mild one, so these are not rates. How we filter them →

What actually reaches you

The label counts the carrier, your body does not

Elemental iron 32.9% of 169.9 g/mol

Absorption Low 25 of 100 · poorly absorbed relative to other forms

Delivered daily 8 mg

Cost per 100 mg delivered $7.48 comparative — uses the absorption score, not a measured percentage

A 500 mg capsule of Iron Fumarate contains 165 mg of iron. How much of that crosses into your blood is not measured for this form — the absorption score ranks it against other forms, it is not a share of the dose. How this is calculated →

Works better with

3 documented

Vitamin C

Dramatically enhances iron absorption

Reduces iron to more absorbable form and prevents oxidation · level A

Folate B12

Essential cofactors for red blood cell formation

Required for DNA synthesis and red blood cell maturation · level A

Copper

Required for iron utilization and transport

Copper needed for ceruloplasmin and iron transport · level A

What not to take with Iron Fumarate

3 documented

Calcium Magnesium Zinc

Can reduce iron absorption when taken together

Competition for divalent metal transporters · moderate significance

Tea Coffee Tannins

Tannins significantly reduce iron absorption

Polyphenols bind iron making it unavailable · high significance

Whole Grains Phytates

Phytic acid in grains can bind iron

Chelation reduces iron bioavailability · moderate significance

Medication interactions

Discuss these with a pharmacist before combining

levothyroxine Major

Iron reduces thyroid hormone absorption

What to do: Take thyroid medication 4+ hours before iron

tetracycline-quinolone-antibiotics Major

Iron significantly reduces antibiotic absorption

What to do: Space doses 2-4 hours apart

carbidopa-levodopa Moderate

May reduce Parkinson's medication effectiveness

What to do: Space doses 2+ hours apart

bisphosphonates Moderate

Iron may reduce bone medication absorption

What to do: Take bone medication 2+ hours before iron

Available forms

bioavailability compared

Ranks 3 of 4 among iron forms. Iron Bisglycinate absorbs better — 90%. Compare all forms →

Ferrous Fumarate (Tablets/Capsules) 25

Ferrous Fumarate (Tablets/Capsules): Better tolerance, Good absorption, Less constipation, High elemental iron

Food sources

before you supplement

Iron fumarate not found naturally in foods Only available as supplement · 25

Red meat (natural heme iron) 2-3 mg per 3 oz serving · 25

Liver (natural heme iron) 18 mg per 3 oz serving · 25

Spinach (natural non-heme iron) 6 mg per cup cooked · 5

Too little

deficiency signs

Iron deficiency anemia
Chronic fatigue and weakness
Pale skin and nail beds
Shortness of breath on exertion
Cold intolerance
Brittle or ridged nails
Restless leg syndrome

Too much

toxicity signs

Mild nausea with high doses
Stomach discomfort (less than sulfate)
Iron overload with excessive intake
Constipation (less common than sulfate)
Drug absorption interference

Who benefits most

iron-sensitive-individuals

Best choice for those who can't tolerate iron sulfate

65-130 mg daily (divided doses)

long-term-iron-supplementation

Superior tolerance for extended supplementation periods

65-100 mg daily

digestive-issues-history

Gentle on stomach for those with GI sensitivities

30-65 mg daily with food

iron-deficiency-anemia-treatment

Effective treatment with better compliance

100-200 mg daily

premium-iron-supplementation

Higher quality iron form for those wanting better tolerance

65-130 mg daily

Where to buy it

compare forms first

Look for Iron Fumarate at 65-130 mg per serving. The form that matters here is Ferrous Fumarate (Tablets/Capsules).

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What else affects it

Beyond other supplements

Tea coffee tannins

Tannins significantly reduce iron absorption

Polyphenols bind iron making it unavailable

Whole grains phytates

Phytic acid in grains can bind iron

Chelation reduces iron bioavailability

How it works

Ferrous (Fe2+) form readily absorbed in duodenum and jejunum
Fumaric acid enhances iron solubility and reduces precipitation
Better tolerance due to improved absorption and reduced gastric irritation
Absorbed via divalent metal transporter with good efficiency
Transported by transferrin to bone marrow for red blood cell production
Stored as ferritin when iron needs are met

Key findings

what the evidence supports

Better tolerated than iron sulfate with similar efficacy
Effective for treating iron deficiency anemia with fewer side effects
Improved patient compliance due to reduced gastrointestinal upset
Good alternative for iron-sensitive individuals

Sources: NIH Examine.com · data reviewed 2026-08-02

Compiled by Denis, who maintains this database and holds no medical qualification — every figure here is sourced or derived, never asserted. How we calculate it →

Common questions

What are the benefits of Iron Fumarate?

Documented effects: Superior digestive tolerance; Good bioavailability and absorption; Effective anemia treatment; Reduced constipation risk. Significantly less nausea and stomach upset than iron sulfate

How much Iron Fumarate should you take per day?

The common daily range is 65-130 mg. By goal — maintenance: 30-65 mg elemental iron daily; therapeutic: 100-200 mg elemental iron daily; gentle supplementation: 30-100 mg elemental iron daily; elemental iron: Contains ~33% elemental iron.

When is the best time to take Iron Fumarate?

With food for comfort. First effects are typically reported within 1-2 weeks for energy and symptom improvements.

What are the side effects of Iron Fumarate?

At excessive doses: Mild nausea with high doses, Stomach discomfort (less than sulfate), Iron overload with excessive intake, Constipation (less common than sulfate), Drug absorption interference. Safety rating at normal doses is 8 out of 10.

What should you not take with Iron Fumarate?

Separate it from Calcium Magnesium Zinc, Tea Coffee Tannins, Whole Grains Phytates by two to four hours. Can reduce iron absorption when taken together

Does Iron Fumarate interact with medications?

Documented interactions with levothyroxine, tetracycline-quinolone-antibiotics, carbidopa-levodopa, bisphosphonates. Discuss these with a pharmacist before combining.

How much Iron Fumarate is too much?

The tolerable upper intake level is 45 mg per day for adults, per NIH ODS. Above that, the risk of adverse effects rises: Mild nausea with high doses, Stomach discomfort (less than sulfate), Iron overload with excessive intake.

How do you know if you need Iron Fumarate?

The marker is Hemoglobin, hematocrit, serum ferritin, transferrin saturation, optimal range Hemoglobin: 12-15.5 g/dL (women), 13.5-17.5 g/dL (men), Ferritin: 15-200 ng/mL. Common deficiency signs: Iron deficiency anemia, Chronic fatigue and weakness, Pale skin and nail beds, Shortness of breath on exertion.

Taken for

74 Digestion

Lactase · Lactobacillus acidophilus · Protease