It ranks first because it combines the preferred bisglycinate form and a research-matched dose with the list's strongest independently verifiable product certification.
NSF Certified for Sport status is independently verifiable in NSF's product database.The 25 mg elemental-iron dose matches a published randomized human trial of ferrous bisglycinate.The label discloses one active ingredient and four excipients: hypromellose, microcrystalline cellulose, leucine and silicon dioxide.Vitamin C is not included.At approximately $0.0107 per milligram of elemental iron, it costs more than twice as much per milligram as Solgar.
It ranks second because its 26 mg dose directly matches the published product-specific pilot and it includes vitamin C, but its testing evidence and value are substantially weaker than Thorne's.
Includes 15 mg vitamin C alongside 26 mg of bisglycinate iron.A published product-specific human pilot used one 26 mg tablet daily for eight weeks.Every vitamin and mineral dose is disclosed, and the product is Non-GMO Project Verified, vegan and kosher.At approximately $0.0192 per milligram, it has the highest elemental-iron cost in this comparison.The published product study was uncontrolled and industry-funded, and no public lot-specific certificate of analysis is linked.
It ranks third because its form, transparency and active-iron value are strong, but the 36 mg dose is less directly matched to the cited 24–25 mg bisglycinate trials and its testing claim lacks public documentation.
Uses Ferrochel ferrous bisglycinate and includes 15 mg vitamin C.All five active nutrients are individually disclosed, with no proprietary blend.Ninety servings provide approximately $0.0093 per milligram of elemental iron.The label says “Third Party Tested,” but no named analytical certification or public product-specific certificate of analysis was located.It is not an iron-only product: each serving also contains 5 mg vitamin B6, 667 mcg DFE folate and 300 mcg vitamin B12.
It ranks fourth despite exceptional value because the fixed scoring system gives much more weight to third-party analytical verification, where Solgar provides little qualifying evidence.
The 25 mg bisglycinate dose matches a published randomized human trial.It has the comparison's lowest cost per elemental-iron milligram at approximately $0.0043.The only listed excipients are microcrystalline cellulose, vegetable cellulose and vegetable magnesium stearate.Vitamin C is not included.The gluten-free and kosher certifications do not verify iron potency, and no USP, NSF or public product-specific certificate was located.
It ranks fifth because its testing documentation is better than most uncertified products, but its form and 20 mg dose have less direct research alignment than the bisglycinate options.
Chewable delivery avoids the need to swallow a capsule or conventional tablet.The label identifies SunActive ferric pyrophosphate rather than hiding the iron form.ISURA is an independent organization using ISO-accredited analytical methods, and the product listing states mass-spectrometry lab testing.Ferric pyrophosphate has less directly comparable supplement-dose research than the 25 mg bisglycinate products.Vitamin C is absent, and the flavored chewable contains xylitol, fruit flavors, citric acid, cellulose, stearic acid and magnesium stearate.
Non-GMO Project Verified; brand states accredited third-party pesticide testing; no public lot CoA
Label states third-party tested; Non-GMO Project Verified and AVA Certified Vegan; no public CoA located
Certified Gluten Free and Kosher Parve; no qualifying potency/purity certification or public CoA located
ISURA non-GMO documentation and mass-spec lab-tested statements; no public lot CoA located
Price
$16
$14.99
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$13.27
How we ranked them
Start with the Supplement Facts panel, not the word “gentle” on the front of the bottle. The important number is elemental iron per serving. A label might mention hundreds of milligrams of an iron compound while supplying a much smaller amount of elemental iron; the products compared here disclose 20–36 mg of elemental iron per serving. Form is the central buying decision. Ferrous bisglycinate is iron chelated to glycine and has more directly relevant human evidence for lower-dose supplementation than conventional ferrous sulfate. A randomized pregnancy trial used 25 mg of elemental iron as ferrous bisglycinate daily and compared it with 50 mg as ferrous sulfate. Another randomized study used 24 mg of elemental iron as bisglycinate in a multinutrient formula. These protocols do not prove that 25 mg is appropriate for every buyer, but they make 24–25 mg bisglycinate products easier to compare with published research. Ferrous sulfate is inexpensive and well studied, but it is not the preferred form for this particular list because gastrointestinal complaints are a common buyer concern. No oral iron form can guarantee freedom from constipation or other gastrointestinal effects. Micronized ferric pyrophosphate, used by Natural Factors, is a different option. Human absorption studies have examined SunActive ferric pyrophosphate in fortified foods, while another trial used 30 mg of micronized ferric pyrophosphate with alpha-lactalbumin. That evidence is less directly transferable to a 20 mg standalone chewable than the 25 mg bisglycinate research is to the bisglycinate products here. Vitamin C can increase non-heme iron absorption under some conditions, but an included 15 mg does not automatically make a product superior. A randomized trial comparing oral iron with and without 200 mg of vitamin C found the two regimens equivalent for its measured outcomes. Products with vitamin C are convenient; products without it can still be reasonable choices. For testing, prioritize a named, independently verifiable program. NSF Certified for Sport verifies the listed Thorne product and is stronger evidence than an unnamed “third-party tested” statement. ISURA uses ISO-accredited analytical methods, although a public lot-specific certificate is still preferable. Non-GMO, vegan, kosher and gluten-free seals answer useful sourcing or dietary questions, but they do not by themselves verify iron potency. Compare value per milligram of elemental iron, not bottle price. Also check capsule versus tablet versus chewable format, fillers, flavors and added vitamins. Avoid proprietary blends that hide active doses, unexplained total-compound measurements and products relying only on “gentle” marketing. Iron dosing should be selected with a physician using relevant laboratory information; supplemental iron can interact with levothyroxine, levodopa and calcium, and accidental overdose is especially dangerous for children. Prices and US iHerb stock were checked on August 5, 2026.
We rank on third-party testing (30%), form and bioavailability (25%), dose versus published human research (20%), label transparency (15%) and value per unit of active ingredient (10%). Specifications are taken from each product's supplement facts panel on its iHerb listing, and certification claims are checked against the certifying body's directory where one is published. We do not test products ourselves, and nothing here is medical advice.
Third-Party Testing
Independent certification or a published certificate of analysis
30%
Form & Bioavailability
Whether the chemical form is the well-absorbed one
25%
Dose vs Research
Per-serving dose versus doses used in published human trials
20%
Label Transparency
Full per-ingredient disclosure and minimal fillers
Which iron form is generally the strongest choice when constipation is a concern?
Ferrous bisglycinate has the most directly relevant evidence among these products. A randomized trial used 25 mg per day and recorded a more favorable gastrointestinal profile than 50 mg ferrous sulfate. That is a group-level finding, not a guarantee: any oral iron form can cause constipation or other gastrointestinal effects in an individual.
How many milligrams of iron should a buyer choose?
Compare elemental iron, not the total weight of the iron compound. Published human studies cited here used 24–25 mg per day for bisglycinate, 26 mg per day in the uncontrolled Blood Builder pilot and 30 mg of micronized ferric pyrophosphate in a combination trial. These are research protocols rather than universal dosing instructions. Appropriate dosing depends on laboratory results, diet, medications and medical context, so it should be discussed with a physician.
Is an iron supplement with vitamin C automatically better?
No. Vitamin C can improve non-heme iron absorption under some conditions, but the two products here include only 15 mg. A randomized trial that used 200 mg vitamin C with each 100 mg iron dose found iron plus vitamin C equivalent to iron alone for its measured outcomes. Included vitamin C is convenient, but it should not outweigh iron form, dose transparency or independent certification.
Can any product honestly promise that it will not cause constipation?
No. Bisglycinate and some alternative forms can have more favorable gastrointestinal results than conventional salts in human studies, but individual tolerance and dose still matter. Higher-dose supplemental iron can cause constipation, nausea or abdominal discomfort. Keep every iron product away from children, and ask a physician or pharmacist about interactions with calcium, levothyroxine, levodopa and other medications.
