Iron Bisglycinate vs Ferrous Sulfate for PCOS: What the Trials Show

What Actually Differs Between the Two Forms?

Ferrous sulfate is a simple iron salt; iron bisglycinate is a ferrous ion bound to two glycine molecules, a chelate structure built to stay intact through the stomach’s shifting pH instead of dissociating into a free ferrous ion right away. That structural difference is also why trials almost never dose the two forms equally — bisglycinate typically appears at 18–25 mg elemental iron a day, well under the 50–60 mg elemental iron a day usual for ferrous sulfate, because the chelated form is marketed as delivering more iron per milligram absorbed.

None of the trials below were run in women with PCOS specifically — they were conducted in Cambodian, Danish and Italian populations with iron deficiency from other causes. That matters less than it sounds: once a tablet is swallowed, an intestinal cell does not know why the person took it. PCOS changes the route to deficiency, mainly heavy or irregular bleeding, not how these two iron salts behave once they reach the gut. Table 1 lays out exactly what four head-to-head trials dosed, in whom, and for how long.

Does Iron Bisglycinate Actually Absorb Better?

In one radio-iron tracer study, ten iron-sufficient men absorbed a geometric mean of 6.0% of iron given as bisglycinate against 1.7% given as ferrous sulfate — a four-fold difference — when both were added to a whole-maize porridge, a meal deliberately rich in phytate, the plant compound that normally blocks a large share of non-heme iron absorption (Bovell-Benjamin et al., 2000). The chelate structure appears to shield the iron from binding to phytate the way a free ferrous ion does. That is a real finding, but read it for what it is: a small, decades-old isotope study in healthy men eating a specific phytate-heavy meal, not a general statement about swallowing either supplement on its own or with an ordinary diet. The same paper found that iron absorption from bisglycinate still rose and fell with the subjects’ own iron status, the same self-regulating pattern seen with any iron source — chelation did not override that.

Does a Lower Bisglycinate Dose Really Match a Higher Sulfate Dose?

This is where the marketing claim is weaker than it sounds, and the best-designed trial says so directly. In the 480-woman Cambodian trial, ferritin after 12 weeks reached 99 µg/L on 60 mg ferrous sulfate against 84 µg/L on 18 mg bisglycinate and 78 µg/L on placebo — a statistically significant gap favoring sulfate (P < 0.001) — and the authors concluded plainly that the lower bisglycinate dose “was not as effective as” ferrous sulfate at raising ferritin (Fischer et al., 2023). That trial was purpose-built and adequately powered to test the exact claim behind chelated iron’s premium price: that you need less of it for the same result.

Milman’s earlier, much smaller Danish trial found the opposite impression — deficiency rates were “low and not significantly different” between 25 mg bisglycinate and 50 mg sulfate (Milman et al., 2014) — but with 40 women per arm and few deficiency events to begin with, that trial was not positioned to detect a real difference even if one existed. Between a small trial finding no difference and a large, purpose-built one finding a difference, the larger trial carries the evidentiary weight. The honest summary: the tolerability advantage below has real trial support; the “equivalent at a lower dose” claim does not hold up against the strongest test it has been given.

Which Form Causes Fewer Gastrointestinal Side Effects?

Bisglycinate produced meaningfully fewer gut complaints in the trials that tested it directly. In the Danish Naestved trial, black stools — a marker of unabsorbed iron passing through the gut — appeared in 8% of women on 25 mg bisglycinate against 31% on 50 mg sulfate, and bisglycinate carried “the most favourable” overall gastrointestinal profile of the three iron forms compared (Milman & Bergholt, 2024). In 24 post-surgical cancer patients with mild anaemia — not a PCOS population, but one of the few trials to grade adverse events directly — grade 1 toxicity occurred in 2 of 12 patients (17%) on bisglycinate against 4 of 12 (33%) on sulfate, though the sulfate dose in that trial was several times higher throughout, which muddies how much of the difference is form versus dose (Ferrari et al., 2012). Pooling nine pregnancy trials, a systematic review found bisglycinate cut the rate of gastrointestinal adverse events by roughly two-thirds against pooled comparator iron forms (incidence rate ratio 0.36, 95% CI 0.17–0.76) — though that comparison lumped together several non-sulfate iron salts, not ferrous sulfate alone, and found no significant hemoglobin or tolerability difference in the four trials it reviewed in children (Fischer et al., 2023).

Why Does This Matter So Much With PCOS Heavy Bleeding?

Tolerability decides whether a course of iron actually finishes, and PCOS supplies a specific reason the course needs to run long enough to work. A skipped cycle followed by an unusually heavy, clot-filled bleed is a recognised PCOS pattern, and it is exactly the kind of recurring iron loss that a single course of supplementation, abandoned after two weeks of nausea, will not catch up on. A form that a person actually keeps taking for the full 8 to 12 weeks a correction typically needs is not a minor convenience here — it is close to the entire point, since the best-tolerated iron in the world does nothing sitting unopened in a cupboard.

Does Diet Change How Much of Either Form You Absorb?

Vitamin C measurably increases non-heme iron absorption, while calcium, tea and coffee measurably reduce it — this is established general iron physiology, not a bisglycinate- or sulfate-specific finding, modelled across meals containing varying amounts of each factor (Hallberg & Hulthén, 2000). The phytate-resistance finding described above is the one place bisglycinate has been shown to behave differently from sulfate against a specific dietary inhibitor; no trial has tested whether chelation also blunts calcium’s or tea’s effect, so extending that protection to every inhibitor would be guessing beyond the evidence. Whether and how to schedule either supplement around meals, coffee or other supplements is a timing question for whoever is managing your iron levels, not a fixed rule to self-apply — the fuller supplement-interaction matrix covers how several PCOS supplements interact with food and with each other in more depth than fits here.

Does Iron Interact With Levothyroxine?

Ferrous sulfate is well documented to reduce levothyroxine absorption when the two are taken close together (Liwanpo & Hershman, 2009), a detail that matters given how often PCOS and hypothyroidism overlap. No dedicated trial has tested whether bisglycinate’s chelate structure changes this specific interaction, and the mechanism — iron cations binding levothyroxine in the gut — involves the iron itself rather than the glycine or sulfate it arrives attached to, so treating both forms with the same caution is the more conservative reading until a trial says otherwise. The fuller diet-and-levothyroxine timing picture covers this alongside calcium, fibre and coffee.

Why Testing Comes Before Either Form

Fatigue, hair shedding and heavy periods can all point toward iron deficiency, but none of them can diagnose it — a symptom list overlaps with dozens of other explanations, and only a blood test separates them. A ferritin and a full blood count are the starting panel; where bleeding is heavy or irregular, understanding why PCOS ferritin can be misleading in either direction is worth reading before assuming a number means what it looks like it means. Supplementing without testing risks the opposite problem entirely: iron overload is a genuine harm, not a theoretical one, and hereditary haemochromatosis is common enough that it is routinely screened for before high-dose iron is started. That screening call belongs to a clinician, not to a supplement label.

Who This Comparison Won’t Help

Neither form fixes bleeding-driven deficiency at its source — if heavy periods are the reason iron keeps running low, replacing the iron without addressing the bleeding means starting over every few months regardless of which form was chosen. Neither form should be started at all if ferritin is already normal or high, which happens often enough in insulin-resistant PCOS on its own. And this comparison says nothing about the right form or dose in pregnancy specifically — several of the trials above happened to be conducted in pregnant women, but that was their trial population, not a recommendation being made here.

You may see PCOS written as polyendocrine metabolic ovarian syndrome (PMOS), after a 2026 global consensus of more than 50 organisations renamed it. Nothing about the iron evidence above changed with the name — this article uses PCOS because that is still what most readers search.

Your Next Step

Ask for ferritin and a full blood count before choosing either form — that result, not a marketing claim about bioavailability, is what should decide the dose and the timeline. If heavy or irregular bleeding is why you are here, that recurring pattern is exactly why finishing the course matters more than which form you start. For how iron sits alongside the rest of what people with PCOS commonly take, the full PCOS supplements section covers the ones with real trial evidence behind them.