What it is
Myo-inositol is a cyclic compound related to glucose, present in membranes and in insulin signalling pathways; the body synthesizes about 4 g of it per day according to a 2026 review. D-chiro-inositol is an isomer of it. People buy it mostly for polycystic ovary syndrome (PCOS), fertility and anxiety.
What the label does not say
- The 40:1 ratio highlighted on bottles rests on a trial of 56 women split into seven groups of 8. The 2023 international PCOS guideline writes that specific types, doses or combinations of inositol cannot be recommended for lack of quality data, and that product quality varies.
- The same guideline classes inositol as an option that may be considered according to preferences, with limited clinical benefits on ovulation, hirsutism or weight, and indicates that metformin should be considered before inositol for hirsutism and central adiposity. For fertility, it calls it an experimental therapy.
- What the trials really show: the 2026 umbrella review (13 meta-analyses) reports a drop in fasting insulin (about 23 pmol/L) and in HOMA-IR (about 1.1) versus placebo, comparable to metformin with fewer digestive effects, but no high-certainty result and 81% low or very low certainty. The 2018 Cochrane review (13 trials, 1,472 women) remains uncertain on live births and clinical pregnancies.
- The trial doses are counted in grams: 2 to 4 g per day for PCOS, 12 to 18 g for anxiety and depressive disorders. A capsule of a few hundred milligrams is not the dose studied.
The forms on the shelf
- Myo-inositol on its own, as a powder: the most studied form for PCOS. Examine puts the trial doses between 200 and 4,000 mg per day, the high dose being the most frequent.
- Myo-inositol and D-chiro-inositol in a 40:1 ratio: the ratio comes from a 2019 trial comparing seven ratios in 56 women, or 8 per group, at 2 g twice a day for 3 months.
- D-chiro-inositol on its own: the 2026 umbrella review reports that myo-inositol, alone or with folic acid, does better than D-chiro-inositol alone.
- Capsules: Examine notes that the doses in the psychiatric trials (14 to 18 g per day of powder) do not translate into capsules.
Published reference values
- No recommended intake or upper limit. Trial doses: 2 to 4 g per day for PCOS, 12 to 18 g for anxiety and depression.
- Examine considers up to 12 g per day well tolerated, with digestive effects at higher doses; the umbrella review reports fewer digestive effects than metformin.
- Health Canada classes inositol under the multivitamin and mineral monograph, with no PCOS-specific claim.
When
The PCOS trials gave it in two daily doses for months; Examine describes taking it before breakfast. Consistency counts more than the hour.
To mention to a professional
- PCOS (diagnosis to confirm and follow up), planned pregnancy or ongoing fertility treatment, diabetes on medication (effect on insulin), anxiety or bipolar disorder on medication, pregnancy and breastfeeding: check with a health care professional. No drug interaction is documented in the sources consulted.
Sources
- Cochrane, Inositol for subfertile women with polycystic ovary syndrome (Showell et al., 2018), plain language summary
- International Evidence-based Guideline for the Assessment and Management of PCOS 2023, summary of recommendations (Monash University)
- Nordio et al., The 40:1 myo-inositol/D-chiro-inositol plasma ratio is able to restore ovulation in PCOS patients: comparison with other ratios, European Review for Medical and Pharmacological Sciences, 2019
- Zhu et al., Effects of inositol in women with polycystic ovary syndrome: an umbrella review of meta-analyses from randomized controlled trials, Frontiers in Endocrinology, 2026
- Examine.com, Inositol
- Health Canada, Natural Health Products Ingredients Database, Inositol
Educational information, drawn from the sources above. It does not replace the advice of a health professional, who remains the reference for your situation, your medications, pregnancy or breastfeeding.