What this means in practice
Nothing changes if you already take myo-inositol. What is worth checking is the myo-inositol number on the label rather than the ratio: the branded 40:1 products used in trials supplied about 1100mg of myo-inositol daily, roughly a quarter of the 4g used in the trials of myo-inositol on its own.
What people say
Myo-inositol and D-chiro-inositol work as a pair, and the ratio is what matters. 40 to 1 is the physiological ratio your body uses, so a 40:1 blend is more effective than myo-inositol on its own, and too much D-chiro-inositol will damage your egg quality.
What the evidence supports
No trial has shown a 40:1 blend to outperform myo-inositol alone. The evidence review commissioned for the 2023 international PCOS guideline found the studies comparing the combination against myo-inositol alone reported no difference in insulin resistance, body mass index or hirsutism score, and rated all of it very low certainty. Only two trials have ever compared myo to D-chiro ratios head to head, and they point opposite ways: one of 56 women, eight per arm, favoured 40:1; a double-blind trial of 60 women found a formulation with roughly eleven times more D-chiro-inositol produced higher pregnancy and live birth rates. The claim that excess D-chiro-inositol harms the ovary comes from a single 54-woman dose-escalation trial that its journal has since flagged with two Expressions of Concern. The guideline's own conclusion is that specific types, doses or combinations of inositol cannot be recommended.
How it is supposed to work
An enzyme converts myo-inositol into D-chiro-inositol, and the proposal is that this enzyme runs too fast in a polycystic ovary, leaving the ovary short of myo-inositol even when whole-body levels look fine. Supplying both in the proportion said to be physiological is meant to correct that imbalance.
Which way it runs
Biochemically the two are related in one direction: the body makes D-chiro-inositol from myo-inositol using an insulin-dependent enzyme, so D-chiro-inositol is the product and myo-inositol the precursor. The retail claim runs the other way, that adding a little D-chiro-inositol back improves what myo-inositol does. It is that second, marketed direction that the trials have failed to show.
What is still missing
The blend is sold as an upgrade over plain myo-inositol, and the handful of trials that compared it against plain myo-inositol found nothing to upgrade.
Sources
- Inositol for Polycystic Ovary Syndrome: A Systematic Review and Meta-analysis to Inform the 2023 Update of the International Evidence-based PCOS GuidelinesFitz V, Graca S, Mahalingaiah S, et al., 2024
- Effects of inositol in women with polycystic ovary syndrome: an umbrella review of meta-analyses from randomized controlled trialsDuan M, Yang M, Li C, Wu X, Yin X, Zhu H, 2026
- The 40:1 myo-inositol/D-chiro-inositol plasma ratio is able to restore ovulation in PCOS patients: comparison with other ratiosNordio M, Basciani S, Camajani E, 2019
- Comparison of the effect of two combinations of myo-inositol and D-chiro-inositol in women with polycystic ovary syndrome undergoing ICSI: a randomized controlled trialMendoza N, Diaz-Ropero MP, Aragon M, et al., 2019
- Update on the combination of myo-inositol/d-chiro-inositol for the treatment of polycystic ovary syndromeLete I, Martinez A, Lasaga I, Centurion E, Vesga A, 2024
- Insulin-sensitising drugs (metformin, rosiglitazone, pioglitazone, D-chiro-inositol) for women with polycystic ovary syndrome, oligo amenorrhoea and subfertilityMorley LC, Tang T, Yasmin E, Norman RJ, Balen AH, 2017
- Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary SyndromeTeede HJ, Tay CT, Laven JJE, et al., 2023
- ClinicalTrials.gov registry sweep: registered myo-inositol and D-chiro-inositol trials in PCOS, and results-posting completenessClinicalTrials.gov (US National Library of Medicine), query run 2026-09-05