Iron and magnesium share an irritating habit. Both can upset digestion, iron by causing constipation and magnesium by loosening stools, and the form on the label influences how much. Choosing between them takes some chemistry and a plain look at what the research can and cannot say.

Iron Without the Constipation

The NIH Office of Dietary Supplements puts the upper limit for adults at 45 milligrams a day from food and supplements combined and says doses at or above that level may cause nausea and constipation. Women 19 to 50 have a recommended allowance of 18 milligrams, which falls to 8 milligrams after 51. CDC data show low ferritin, an early marker of iron deficiency, rose from about 13 percent to about 18 percent among females 12 to 49 between 2003 and 2004 and 2021 to 2023, and CDC notes that inflammation can raise ferritin and mask a shortage.

Form changes tolerability somewhat. A meta-analysis of 43 randomized trials with 6,831 adults found that ferrous sulfate raised the odds of gastrointestinal side effects versus placebo, with an odds ratio of 2.32. NIH says iron amino acid chelates, carbonyl iron, heme iron polypeptides, and polysaccharide-iron complexes “might have fewer gastrointestinal side effects than ferrous or ferric salts,” though it does not name bisglycinate. One trial of 80 pregnant women found that 25 milligrams of iron as bisglycinate caused fewer GI complaints than 50 milligrams as sulfate, but the doses differed by half.

Timing and pairing matter as well. NIH says vitamin C, meat, poultry, and seafood improve absorption of non-heme iron, while phytate and polyphenols reduce it, and some levothyroxine labels advise separating that drug from iron by four hours. Alternate-day dosing raised fractional absorption to 21.8 percent from 16.3 percent in iron-depleted young women taking 60 milligrams, though that trial measured absorption rather than constipation. For people with hereditary hemochromatosis, NIH notes that the American Association for the Study of Liver Diseases recommends avoiding iron and vitamin C supplements.

Check the Label and the Seal

The sources reviewed here do not rank products, so label details do the work: the elemental amount per serving, the form, and a third-party seal. Fullscript’s dietary supplement quality guide, written by Karolyn Gazella and medically reviewed by Alex Keller, ND, states that for finished products, “NSF and USP are the leading organizations doing this testing.”

U.S. Pharmacopeia says its verification involves a quality systems review, an on-site audit, and product testing for labeled ingredient amounts, dissolution, and contaminants. Its FAQ adds that USP does not comprehensively address efficacy. NIH likewise says seals “do not guarantee that a product is safe or effective.”

Magnesium Glycinate Versus Citrate

NIH’s magnesium fact sheet says high supplemental doses often cause diarrhea, with the carbonate, chloride, gluconate, and oxide forms reported most often, and it sets the supplemental upper limit at 350 milligrams. Citrate dissolves and absorbs better than oxide. MedlinePlus classifies the drug-strength citrate product as a saline laxative that usually produces a bowel movement within 30 minutes to 6 hours and cautions against using it for more than a week without a doctor’s direction.

NIH’s fact sheet does not address glycinate, so claims that it is gentler lack backing from that source. For anyone shopping for the best magnesium glycinate supplement, the label checks above are the available criteria: elemental magnesium per serving, a supplemental total under the 350 milligram ceiling, and a verification seal.