Non-Constipating Iron: Gentler Forms, Doses, Alternate Days

By Erin Rose · Published September 21, 2026 · Reviewed against primary sources · Methodology · About Us

Not medical advice. This summarizes the randomized trials that compared iron forms, doses and schedules for gastrointestinal side effects, the labels manufacturers file with the NIH, and current Amazon prices. Iron deficiency is diagnosed with a ferritin test, and a treatment dose is a clinician’s call.

Every gentle iron we track, ranked by cost per 25 mg of elemental iron

Product links go to Amazon and we may earn a commission if you buy. It never changes which product we pick or what we say about it. How we choose.

25 mg is the dose the bisglycinate pregnancy trial used and the median bisglycinate label delivers, so it is the unit of comparison. Cost per 25 mg is the current Amazon price divided by the elemental iron in the pack; the alternate-day column is that dose 3.5 times a week. Form and mg per unit are read from each label. The “gentleness basis” column is what the trials say about that form, not a score.

Iron bisglycinate capsules and liquid irons we track, cheapest per 25 mg elemental iron first, with the chemical form, mg per unit, units needed for 25 mg, weekly cost on alternate days, and the trial basis for calling the form gentle.

Product	Per 25 mg	Per week, alternate days	Form	mg per unit	Units for 25 mg	Gentleness basis	Price / count	Buy
NOW Foods Iron 36 mg Double Strength	$0.074	$0.26	iron bisglycinate (chelate)	36 mg per capsule	0.7	bisglycinate: fewer GI complaints than sulfate in the pregnancy trials	$9.57 / 90	Check price
Thorne Iron Bisglycinate 25 mg	$0.267	$0.93	iron bisglycinate (chelate)	25 mg per capsule	1	bisglycinate: fewer GI complaints than sulfate in the pregnancy trials	$16.00 / 60	Check price
MegaFood Blood Builder	$0.310	$1.08	iron bisglycinate (chelate)	26 mg per tablet	1	bisglycinate: fewer GI complaints than sulfate in the pregnancy trials	$28.99 / 90	Check price
Nature's Way Liquid Iron 18 mg	$0.395	$1.38	ferric glycinate, liquid	18 mg per serving	1.4	ferric glycinate: no head-to-head GI trial on file	$9.10 / 32	Check price
MegaFood Blood Builder Liquid Iron	$1.028	$3.60	iron bisglycinate, liquid	8 mg per serving	3.1	bisglycinate: fewer GI complaints than sulfate in the pregnancy trials	$24.99 / 76	Check price
Floradix Iron + Herbs	$1.529	$5.35	ferrous gluconate, liquid	10 mg per serving	2.5	gluconate: no head-to-head GI trial on file	$30.59 / 50	Check price
Pure Encapsulations Iron Liquid	$2.674	$9.36	ferric pyrophosphate, liquid	15 mg per serving	1.7	ferric pyrophosphate: no GI trial on file	$38.50 / 24	Check price

The spread is $0.074 to $2.674 per 25 mg, 36.2× from cheapest to priciest. The shaded rows are the literal answer, bisglycinate at 30 mg or less per unit: Thorne Iron Bisglycinate 25 mg and MegaFood Blood Builder. NOW Foods Iron 36 mg Double Strength is the cheapest iron here but its capsule is 36 mg, above the trial dose and most of the way to the 45 mg limit, which is why it is not shaded. The 4 liquids cost more per milligram for the adjustable spoon; Enfamil Fer-In-Sol Iron Drops (ferrous sulfate) is an infant product dosed by a clinician per kilogram and is not ranked.

Form: the sulfate meta-analysis and the bisglycinate trials

Constipation, nausea and dark stools come from unabsorbed iron left in the gut, and the reference form has the reference side-effect record. A meta-analysis of 43 randomized trials in 6,831 adults found ferrous sulfate raised the odds of gastrointestinal side effects 2.32-fold against placebo and about 3-fold against intravenous iron, with no relationship between the odds and the dose (Tolkien 2015, PMID 25700159). Bisglycinate is iron chelated to two glycine molecules and absorbed largely intact; the evidence for it is real and narrower than the marketing. The systematic review of 17 randomized trials found ferrous bisglycinate produced higher hemoglobin and fewer GI adverse events than other iron supplements in pregnant women (incidence rate ratio 0.36), and no significant difference in children (Fischer 2023, PMID 36728680). The trial behind that pregnancy result randomized 80 Danish women to 25 mg of iron as bisglycinate or 50 mg as ferrous sulfate from mid-pregnancy to delivery: the same protection against iron deficiency and anemia, fewer GI complaints on bisglycinate (P = 0.001) (Milman 2014, PMID 24152889).

The same group’s pooled report of 482 pregnant women is the most useful single reading for the dose question: across 20 to 60 mg of iron as fumarate there was no association between dose and GI complaints, 80 mg brought more constipation and laxative use, and of three equipotent prophylactic doses, 25 mg bisglycinate had the most favourable GI profile and the fewest black stools (8%, against 22% on 40 mg fumarate and 31% on 50 mg sulfate) (Milman 2024, PMID 39582678). The sulfate vs bisglycinate comparison walks the absorption trials; the summary is that bisglycinate lets you take half the milligrams for the same result, and the smaller dose spares your gut as much as the chelate does.

Dose and schedule: every other day absorbs more and irritates less

An oral iron dose raises the hormone hepcidin for about 24 hours, which shuts the absorption gate on the next dose (Moretti 2015, PMID 26289639). That is why the Zurich group tested alternate days. In 40 iron-depleted women, 60 mg on alternate days was absorbed 21.8% cumulatively against 16.3% on consecutive days, and a single morning dose beat a split dose (Stoffel 2017, PMID 29032957). In 19 women with iron-deficiency anemia, 100 or 200 mg on alternate days was absorbed 40–50% better per dose than on consecutive days (Stoffel 2020, PMID 31413088). The double-blind placebo-controlled trial that settled the side-effect question randomized 150 iron-depleted women to 100 mg daily for 90 days or the same dose on alternate days for 180: ferritin was identical at equal total iron, GI side effects on intake days were 1.56 times as frequent on the daily schedule, hepcidin was lower on alternate days, and iron deficiency at six months was 3% against 11.4% (von Siebenthal 2023, PMID 38021373). A 2024 systematic review of 594 participants found no hemoglobin difference between schedules and fewer nausea, taste and bowel complaints on alternate days (Kamath 2024, PMID 37979057), and the American Gastroenterological Association’s 2024 practice update says once a day at most, with every-other-day dosing better tolerated for some patients (DeLoughery 2024, PMID 38864796).

Dose is the other half. The RDA is 18 mg a day for menstruating women and 8 mg for men and postmenopausal women, the upper limit for routine use is 45 mg, and a treatment dose is set by a clinician after a ferritin test. The rule that falls out of the trials: the smallest dose that moves your ferritin, on alternate mornings, away from calcium and coffee (the timing page).

Liquid iron: gentler because the spoon sets the dose

Liquid iron is not a different iron; it is a smaller, adjustable dose of whichever salt is in the bottle. Of the 63 liquid iron labels we read, the median delivers 10 mg a serving and three-quarters direct 20 mg a day or less; 58.7% are aimed at children. 77.8% declare a form, and of those 24.5% are bisglycinate, with the rest split across gluconate, sulfate, ferric salts and polysaccharide iron. The GI record belongs to the form: a gluconate liquid carries a ferrous salt’s profile at a lower dose, a bisglycinate liquid the chelate’s. MegaFood Blood Builder Liquid Iron is the bisglycinate liquid we track, dosed from 8 mg per half-teaspoon up the label’s own scale; Nature's Way Liquid Iron 18 mg is the cheapest liquid per milligram at 18 mg a tablespoon as ferric glycinate, a form with no head-to-head GI trial on file. The liquid iron page ranks every liquid we carry by cost per milligram and per day.

We read 2,004 iron labels: the gentle form is dosed lower, and nothing is dosed every other day

Original research2,004 on-market iron labels from the NIH Dietary Supplement Label Database on 2026-08-29, 1,592 with an iron row and 1,961 with a direction, from 459 brands; a separate read of 168 labels whose iron row names a bisglycinate, Ferrochel or chelate form (145 of them pure bisglycinate); and 63 liquid iron labels. Every figure was recomputed in two halves and only the ones that agreed are printed.

Every one of the 1,961 labels with a direction says take it daily; none says every other day. The median directed dose is 18 mg, and 68.5% of the 1,592 dosed labels sit at or below the 18 mg RDA. The bisglycinate and chelate labels are dosed below ferrous sulfate: a median 18 mg a serving across 417 labels against 28 mg across 108 sulfate labels. Among the 168 bisglycinate labels, the median serving is 25 mg (15 to 28 mg across the middle half) and the median unit is 25 mg; 73.8% deliver at least the 18 mg RDA, 26.2% need two or more units for a serving, only 3.6% direct more than the 45 mg limit a day, and only 5.4% put the word “gentle” on the label. The shelf already sells the dose the trials used; what it does not sell is the schedule.

Which gentle iron to buy

The trial dose, verified

Thorne Iron Bisglycinate 25 mg $0.267 per 25 mg

25 mg of iron bisglycinate per capsule, the dose the Danish pregnancy trial used, NSF Certified for Sport. One capsule every other morning is $0.93 a week. 3.6× the price of NOW Foods Iron 36 mg Double Strength per milligram, for a capsule that is the right size.

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Cheapest bisglycinate per milligram

NOW Foods Iron 36 mg Double Strength $0.074 per 25 mg

36 mg Ferrochel bisglycinate per capsule, 90 capsules for $9.57. The capsule is above the 25–30 mg answer and most of the way to the 45 mg limit, so it suits a clinician-set treatment dose on alternate days, not a gentle maintenance dose.

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Adjustable low dose, bisglycinate

MegaFood Blood Builder Liquid Iron $1.028 per 25 mg

8 mg per half-teaspoon, scaling up the label’s own directions; 76 half-teaspoon servings for $24.99. The only bisglycinate liquid we track, at 3.9× the capsule’s price per milligram for the dose control.

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Every bisglycinate we track is ranked per 25 mg on the best bisglycinate page, and the best iron overall covers the sulfate and fumarate products this page leaves out on purpose.

Frequently asked questions

Related guides

• Iron: what to buy and how much
• Best iron bisglycinate, ranked per 25 mg
• Ferrous sulfate vs bisglycinate
• Liquid iron, ranked per milligram
• Iron dosage guide
• Best time to take iron
• The ferritin test
• Best iron supplement, ranked

Sources