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Supplements

Inositol for PCOS: dose, forms, and the 40:1 ratio

Inositol is the most-studied supplement for PCOS (now officially PMOS), and the only one the 2023 international guideline names by choice. Here is the form that was tested, the dose, and how strong the evidence actually is.

Evidence-based · Sources cited · Updated July 2026

Take myo-inositol and D-chiro-inositol in a 40:1 ratio, roughly 4g of myo per day. In trials it lowers fasting insulin and free testosterone and improves ovulation, with results comparable to metformin but far fewer side effects. Give it about 3 months. It is the only supplement the 2023 PCOS guideline names, though it grades the evidence low.

What the guideline actually says

The 2023 International Evidence-Based Guideline for PCOS says inositol could be considered.[1] That wording is deliberate. The panel judged the harm to be low and the possible metabolic benefit real, but it graded the quality of evidence as low. So this is a reasonable, low-risk thing to try. It is not a proven cure, and consumer marketing tends to overstate the certainty here.

The forms and the 40:1 ratio

The version studied in PCOS is a mix of two inositols: myo-inositol and D-chiro-inositol, combined in a 40:1 ratio. That ratio is not arbitrary. It mirrors the ratio the two forms sit in naturally in human plasma. The two do different jobs. Myo-inositol helps your cells take up glucose. D-chiro-inositol helps the body build glycogen. Together, at 40:1, they act on the insulin signalling that drives most PCOS.[3]

What the trials show

Randomised trials report that inositol lowers fasting insulin, lowers HOMA-IR (a marker of insulin resistance), and lowers free testosterone, while improving ovulation.[3] Head-to-head against metformin, the effect on insulin and ovulation looks comparable, but inositol is much better tolerated: mild digestive upset at most, versus the nausea metformin is known for.[2] The honest caveat is that these trials are small and use different doses, which is why the guideline stays cautious.[1]

How to take it

The typical studied dose is about 2g of myo-inositol plus 50mg of D-chiro-inositol, twice a day. That works out to roughly 4g of myo-inositol daily, in the 40:1 ratio. Give it about 3 months before you decide whether it is working, because the markers move slowly. It is well tolerated. The most common complaint is mild stomach upset, which usually settles.[3] If you want to know whether it is actually moving your numbers, retest fasting insulin and testosterone rather than going on feel.

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Sources

  1. [1] 2023 International Evidence-Based Guideline for the Assessment and Management of PCOS. PMC10505534
  2. [2] Greff D. et al., inositol vs metformin in PCOS, randomised trial evidence. PMC9786218
  3. [3] Inositol in PCOS, updated review. Gynecol Endocrinol 2024

This article is educational and is not medical advice. Talk to a clinician before starting, stopping, or combining treatments.