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Myo-inositol: what it does and what the PCOS research shows

By the Life in Good Form editors. Updated 2026-10-15.

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What myo-inositol is

A 2026 scoping review in Nutrients describes myo-inositol (MI) as a widespread compound with a vital role in insulin signaling pathways. That link is why it is studied for PCOS, a common hormone disorder in women of childbearing age marked by irregular ovulation, high androgens and metabolic problems including insulin resistance.

Supplements sell myo-inositol alone or combined with D-chiro-inositol, another form. NOW Foods' Inositol Powder, for example, gives 730 mg per quarter teaspoon with no other ingredients.

What the PCOS research shows

The 2026 umbrella review in Frontiers in Endocrinology combined 13 meta-analyses of randomized trials. Of 85 evidence items, 18.9% were moderate quality, 40% low and 41.1% very low. The authors called inositol a promising therapy but warned that low-quality results should not solely guide clinical decisions.

A 2026 meta-analysis in Clinical Endocrinology of 9 trials with 440 women found inositol reduced total testosterone and free androgen measures and raised SHBG. The most consistent response was in normal-weight women, suggesting body type affects results.

Doses, the 40:1 ratio and how long to take it

The 2026 scoping review reports that standard daily doses of myo-inositol in studies range from 2 g to 4 g, taken alone or combined with D-chiro-inositol in a 40:1 ratio. The umbrella review explains that this ratio mirrors the roughly 40:1 balance of the two forms in healthy women's blood, and that the ratio is disrupted in PCOS. The scoping review adds that the 40:1 ratio still needs large trials and pharmacokinetic studies to confirm it.

Treatment length matters. The scoping review says at least 24 weeks may be needed for significant hormone changes such as higher SHBG. The body also makes about 4 g of inositol a day on its own, mainly in the kidneys, and inositol is found in grains and seeds.

Inositol compared with metformin

Metformin, a diabetes medicine, is often used in PCOS but commonly causes stomach side effects. The umbrella review found inositol had a lower risk of adverse events and suggested it may be an alternative or add-on for people who cannot tolerate metformin or who are planning pregnancy. The scoping review likewise found no severe adverse events or long-term toxicities in the protocols it reviewed. Any switch or combination should be decided with your doctor.

Limits of the evidence

• Quality. Over half of the meta-analyses in the umbrella review were rated low or very low quality on a standard tool.
• Variable studies. The 2026 scoping review of 13 studies said benefits should be interpreted cautiously because study designs vary.
• Different forms. The umbrella review noted that inositol subtypes may differ in effect, which could inform personalized treatment.
• Clinical outcomes. The Clinical Endocrinology authors called for larger trials measuring hirsutism, ovulation and patient-reported results.

Safety

• Talk with your doctor before using inositol for PCOS or fertility, so it fits with other treatment.
• Pregnancy and medicines. NOW Foods advises consulting a physician if pregnant, nursing, taking medication or with a medical condition.
• Not a replacement. PCOS involves metabolic risks that need medical follow-up, not supplements alone.
• Blends. Hormone balance products may add herbs or vitamins with their own cautions.

How to choose a myo-inositol supplement

• Form. Plain myo-inositol, or a myo-inositol and D-chiro-inositol combination. Many combination products advertise a 40:1 ratio; ask your doctor which suits you.
• Powder or capsules. Powders such as NOW Foods' dissolve in water; NOW Foods directs a quarter teaspoon daily.
• Testing. Choose brands that state third-party certification, such as NSF.
• Single purpose. Skip broad hormone blends if you want to match the inositol studies.
• Give it time with your care team and track cycles and lab results.

What to look for when you buy

Questions people ask

What does myo-inositol do?

It plays a role in insulin signaling. In PCOS studies, inositol lowered LH and testosterone and improved ovulation compared with placebo.

Does myo-inositol help PCOS?

Evidence is promising but not high quality. A 2026 umbrella review found moderate-quality evidence for hormone, insulin resistance and ovulation improvements.

Myo-inositol or D-chiro-inositol?

Both are studied. The umbrella review said subtypes may differ in effect; ask your doctor which form fits you.

Who responds best to inositol?

A 2026 meta-analysis found the most consistent testosterone reduction in normal-weight women.

Is myo-inositol safe in pregnancy?

Ask your doctor. NOW Foods advises consulting a physician if pregnant or nursing.

Sources

• Effects of inositol in women with polycystic ovary syndrome: an umbrella review of meta-analyses from randomized controlled trials (pmc.ncbi.nlm.nih.gov), Frontiers in Endocrinology (PMC12932251)
• Metabolic Phenotype Predicts Biochemical Response to Inositol Supplementation in Polycystic Ovary Syndrome (pmc.ncbi.nlm.nih.gov), Clinical Endocrinology (PMC13326987)
• Effect of Myo-Inositol Supplementation in Polycystic Ovary Syndrome: Scoping Review (pmc.ncbi.nlm.nih.gov), Nutrients (PMC13363645)
• Inositol Powder (www.nowfoods.com/products/supplements/inositol-powder), NOW Foods

Sources checked 2026-10-15.