Ferrous sulfate is the cheap, long-established iron salt that clinicians have prescribed for decades, and iron bisglycinate is a chelated form marketed as easier on the stomach. Our verdict: bisglycinate has some published evidence for fewer digestive side effects, ferrous sulfate has the longer track record and the lower price, and the more important question is whether you should be taking iron at all. Iron is one of the few supplements where too much is genuinely dangerous, so the decision to start, and the form, belongs with a clinician who has seen your blood work.
We compared the two forms on published research, label conventions, cost and known interactions, using the NIH Office of Dietary Supplements iron fact sheet and a 2023 meta-analysis in Nutrition Reviews as anchors. We are reviewers, not hematologists, and nothing here is a recommendation to take iron or advice on how much. Fatigue has many causes besides low iron, and only a blood test can tell you which applies to you.
What separates the two forms
Ferrous sulfate is an iron salt. It dissolves and releases iron in the gut, where it is absorbed, and it is the reference form many studies use. A common 325 mg ferrous sulfate tablet provides about 65 mg of elemental iron, which is why reading labels for the elemental amount matters more than the compound weight.
Iron bisglycinate, often sold under the Ferrochel ingredient name, binds iron to two molecules of the amino acid glycine. The chelate is designed to be absorbed through a different route and to leave less free iron irritating the stomach lining. It is usually more expensive per milligram of elemental iron and appears in brands like Thorne, whose Iron Bisglycinate lists 25 mg of elemental iron per capsule, compared with the 65 mg in a typical ferrous sulfate tablet. Verify current labels, since formulas change.
| Factor | Iron bisglycinate | Ferrous sulfate |
|---|---|---|
| Type | Amino acid chelate | Iron salt |
| Track record | Newer, smaller trial base | Decades of clinical use, often the reference form |
| Digestive side effects | Fewer reported in a 2023 meta-analysis | Nausea, constipation and stomach upset commonly reported |
| Typical elemental iron per unit | Often 18 to 36 mg (check the label) | About 65 mg in a 325 mg tablet |
| Cost | Higher per mg of iron (verify current pricing) | Among the cheapest supplements sold |
What the research says about tolerability
The clearest data point is a 2023 systematic review and meta-analysis in Nutrition Reviews (Fischer and colleagues) that pooled 17 randomized trials comparing ferrous bisglycinate with other iron supplements. In pregnant women, bisglycinate was associated with higher hemoglobin after 4 to 20 weeks and with fewer gastrointestinal adverse events than other iron forms. In children, no significant differences were found. The authors called for more and better trials, and the ingredient's manufacturer publicized the results, which is worth knowing when you read trade coverage of them.
That is a meaningful signal, because stomach upset is a main reason people stop taking iron. But a lower side-effect rate in pooled trials does not mean bisglycinate will suit every person, and many of the studies were small. Ferrous sulfate remains effective at raising iron stores, and some people tolerate it well.
Other iron forms you will see on the shelf
Bisglycinate and sulfate are not the only choices, and the label compound changes how much iron you actually get. The NIH Office of Dietary Supplements lists the approximate elemental iron share of common salts: ferrous fumarate is about 33 percent iron, ferrous sulfate about 20 percent and ferrous gluconate about 12 percent. That is why two tablets with the same milligram number on the front can deliver very different amounts of iron.
You will also see polysaccharide iron complex, heme iron polypeptide made from animal sources, and carbonyl iron in some products, each with its own smaller body of research. Intravenous iron and prescription oral forms exist for people who cannot absorb or tolerate tablets, and those are entirely a clinician's call. If a product hides its iron form inside a proprietary blend, you cannot compare it properly, which is a reason to skip it.
Why iron is a clinician decision, not a shelf decision
Iron deficiency is common, particularly in people who menstruate, are pregnant, follow some vegetarian diets or have conditions affecting absorption. But iron overload is real too. Hereditary hemochromatosis causes the body to absorb and store too much iron, and supplements can worsen the damage it does to the liver, heart and pancreas. Symptoms of low iron, such as tiredness, overlap with many other conditions, so guessing is a poor strategy.
Low iron can also be a sign of something else, such as blood loss from the digestive tract, that needs investigating rather than masking. That is one reason guidelines push clinicians to find the cause, not just replace the iron. A ferritin and complete blood count are inexpensive tests, and your doctor can decide what form fits once the result is in.
US iron supplements carry an FDA-required warning that accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6. Keep every bottle, including gummies and prenatal vitamins, locked away from children.
Interactions to raise with a pharmacist
Iron can reduce absorption of levothyroxine for thyroid conditions, some antibiotics (tetracyclines and fluoroquinolones) and certain Parkinson's and osteoporosis medicines. Antacids and proton pump inhibitors can lower iron absorption in turn, as can calcium supplements, and tea or coffee taken at the same time. Vitamin C is known to increase iron absorption. Spacing medicines apart is often the answer, but the exact timing is something a pharmacist should set for your specific list.
If you already take a multivitamin, check how much iron it contains before adding more. Our multivitamin vs individual vitamins comparison and best multivitamin guide explain how to read stacked labels, and the supplements hub covers how we judge third-party testing. Budget readers will find ferrous sulfate from brands covered in our Nature Made review.
The bottom line: bisglycinate looks gentler on the stomach and ferrous sulfate is cheaper and well proven, but confirm with a blood test and your doctor that you need iron before choosing either form.
