What Is Inositol? Myo, D-Chiro and What the Trials Used

Inositol is a sugar-like molecule that your kidneys make in gram quantities every day and that fruit, beans and grains supply; the two forms sold as supplements, myo-inositol and D-chiro-inositol, are chemical cousins that help carry insulin's signal inside cells. Its evidence is almost entirely in polycystic ovary syndrome: across 26 trials of 1,691 women, inositol made a regular cycle 1.79 times as likely as placebo, matched metformin on that outcome and caused far fewer stomach upsets, yet the review written for the 2023 international PCOS guideline called the evidence "limited and inconclusive".^{1, 2, 3} Most trials used 4 g of myo-inositol a day for three to six months; the much-quoted 40:1 ratio comes from two small Italian trials.^{4, 5} PCOS is a GP diagnosis; the NHS now calls it polyendocrine metabolic ovarian syndrome (PMOS), previously PCOS.¹² UK sachets vary, and this page sets what they contain against what the trials used.

What is inositol?

Inositol is a six-carbon ring that looks like glucose; it was once called vitamin B8, but the body makes its own, so it is not a vitamin. Nine arrangements exist; myo-inositol is the one in food and in almost every cell, and it is the backbone of the phosphoinositides that relay signals from hormone receptors to the inside of the cell, including insulin's. Wholegrains, citrus fruit, beans, nuts and seeds supply roughly a gram a day; supplements add two to four grams, which is why trial doses are measured in grams.

What is the difference between myo-inositol and D-chiro-inositol?

D-chiro-inositol is made from myo-inositol inside tissues by an enzyme that insulin switches on. The two then do different jobs: myo-inositol-based messengers drive glucose uptake and, in the ovary, help follicle-stimulating hormone work, while D-chiro-inositol-based messengers drive glycogen storage and, in the ovary, androgen production. Blood carries them at roughly 40 parts myo to 1 part D-chiro, and the theory behind the combined products is that the ovaries of women with PCOS convert too much myo to D-chiro, so the balance is restored by supplying both in the blood's ratio. That is a hypothesis with trial support of the size set out next, not a settled mechanism.

What did the 40:1 trials actually show?

Two trials from one Rome group. In 2012, 50 overweight women with PCOS took either myo-inositol alone or the 40:1 combination for six months: both groups improved on metabolic measures, with no difference at six months and a faster response in the combination group at three.⁴ In 2019, 56 women were split eight to a group across seven ratios, from D-chiro alone to 80:1, at 2 g of inositols twice a day for three months; ovulation was restored most often at 40:1, and formulations with more D-chiro did worse, not better.⁵ Eight women per arm is a very small trial, and no independent group has repeated the comparison; the ratio is plausible, and the certainty is low.

Does inositol work for PCOS? The numbers

The strongest summary is the 2023 meta-analysis of 26 randomised trials in 1,691 women: inositol versus placebo made a regular menstrual cycle 1.79 times as likely (confidence interval 1.13 to 2.85), lowered BMI by 0.45, lowered free and total testosterone, fasting glucose and the insulin response to a meal, and raised sex hormone-binding globulin; against metformin it was non-inferior for cycles.¹ Nine earlier trials pooled in 2017 found fasting insulin fell by a standardised 1.02 and insulin resistance (HOMA) by 0.59.⁶ Two head-to-head meta-analyses found no difference between myo-inositol and metformin on insulin, HOMA, testosterone or BMI, with stomach side effects 5.17 times as common on metformin.^{7, 8} Adding inositol to metformin, in six trials of 388 women, made regular cycles 1.56 times as likely and reduced hirsutism scores.⁹

What are the limits of the evidence?

The review commissioned for the 2023 international guideline pooled 30 trials of 2,230 women and concluded the evidence is "limited and inconclusive": benefits for some metabolic measures and perhaps ovulation, metformin possibly better for waist-to-hip ratio and hirsutism, no clear difference for reproductive outcomes.² The guideline keeps lifestyle change and metformin at the centre and lists inositol as an option with limited clinical benefit; the NHS lists the same medicines.^{3, 12} A 2026 umbrella review of 13 meta-analyses found no high-quality evidence anywhere, moderate-quality evidence for testosterone, SHBG, insulin resistance and ovulation, and ovulation 2.75 times and live birth 2.29 times as likely versus placebo on lower-quality data.¹⁰ The Cochrane review of subfertile women called the live-birth effect uncertain (odds ratio 2.42, interval 0.75 to 7.83).¹¹ So: real effects on cycles and insulin measures, uncertain effects on babies, and three to six months before anyone can judge. What to eat, and the medicines that act on insulin, are covered on food for PCOS.

Does inositol help with weight, hair or anything else?

Weight: the pooled BMI change against placebo was 0.45 units, a kilogram or so for an average-height woman over several months, within trials of women with PCOS; there is no trial of inositol for weight in anyone else, and no trial of belly fat specifically.¹ Hair: no trial has measured hair growth; the hirsutism (unwanted hair) finding above is the opposite problem.⁹ Without PCOS: for a woman with regular cycles and normal insulin no trial has looked for a benefit, and none has found harm beyond the stomach effects below.

Is there any downside, and who should not take inositol?

Across the trials inositol was well tolerated; the stomach effects (nausea, wind, loose stools) that appear at higher doses were far rarer than on metformin.^{2, 7} Three groups take care. Anyone on antidiabetic medicines, including metformin and insulin, asks their GP or diabetes team first, because inositol lowers glucose and insulin and the combination has only been trialled under supervision.⁹ Anyone pregnant, breastfeeding or in fertility treatment tells the midwife or clinic, since the live-birth evidence is uncertain and some combined sachets carry botanicals that are not for pregnancy.¹¹ Under-18s should not take adult sachets. Ask a GP or pharmacist before combining it with glucose-lowering medicines; otherwise take it with water at the same time each day and give it three cycles. What the trials measured, over three to six months, was a cycle that arrives on a more predictable schedule and glucose, insulin or testosterone results that move at a blood test.¹

How much did the trials use, and what do UK sachets contain?

Most trials used 2 g of myo-inositol twice a day, 4 g in total; the 2019 ratio trial used 2 g of inositols twice a day; the metformin comparisons used 4 g a day of myo-inositol.^{1, 5, 7} UK sachets and capsules range from a few hundred milligrams to 4 g, so the label, not the word "inositol", tells you where a product sits against the trials. Many UK sachets supply 2 g of myo-inositol each with a small amount of D-chiro-inositol, so matching the 4 g a day the trials above used means two sachets; where minerals are added, chromium contributes to normal macronutrient metabolism and to the maintenance of normal blood glucose levels, zinc contributes to normal fertility and reproduction, and magnesium contributes to the reduction of tiredness and fatigue.¹ Read the label's cautions too: these blends are typically not for under-18s or during pregnancy, used in breastfeeding only under medical supervision, unsuitable for anyone with an oestrogen-related condition where a herbal extract is included, and anyone on medication, including antidiabetic medicines, asks a GP first.

The wider evidence for inositol's companions, from vitamin D to spearmint, is graded on which PCOS supplements have evidence.

The Wellgard women's health guide begins with what causes BV and why it comes back.

Frequently asked questions

Which type of inositol is best for PCOS?

Myo-inositol has most of the trials, at 4 g a day.^{1, 2} The 40:1 combination has two small trials from one group suggesting it restores ovulation faster; D-chiro-inositol on its own did worse at reproductive outcomes in the ratio trial.^{4, 5}

How long should you take inositol for PCOS?

The trials ran three to six months, and cycle regularity was judged over that span; a fair test is three cycles.^{1, 9} Nothing in the trials suggests a point at which it stops working or must be stopped.

What happens if you take inositol every day?

Daily is how every trial used it. The commonest effects were none; some women notice loose stools or wind at 4 g, which eases at a lower dose.^{2, 7}

What happens if you take inositol without PCOS?

No trial has measured an effect in women with regular cycles: the cycle, insulin and hormone findings all come from women with PCOS.^{1, 10}

Is inositol good for hair growth?

No trial has studied it. The only hair finding is less unwanted facial and body hair when inositol is added to metformin.⁹

Sources

1. Greff D, et al. Inositol is an effective and safe treatment in polycystic ovary syndrome: a systematic review and meta-analysis of randomized controlled trials. Reproductive Biology and Endocrinology, 2023. PubMed 36703143.
2. Fitz V, et al. Inositol for polycystic ovary syndrome: a systematic review and meta-analysis to inform the 2023 update of the international evidence-based PCOS guidelines. Journal of Clinical Endocrinology and Metabolism, 2024. PubMed 38163998.
3. Teede HJ, et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. Human Reproduction, 2023. PubMed 37580037.
4. Nordio M, Proietti E. The combined therapy with myo-inositol and D-chiro-inositol reduces the risk of metabolic disease in PCOS overweight patients compared to myo-inositol supplementation alone. European Review for Medical and Pharmacological Sciences, 2012. PubMed 22774396.
5. Nordio M, 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. PubMed 31298405.
6. Unfer V, et al. Myo-inositol effects in women with PCOS: a meta-analysis of randomized controlled trials. Endocrine Connections, 2017. PubMed 29042448.
7. Facchinetti F, et al. Short-term effects of metformin and myo-inositol in women with polycystic ovarian syndrome: a meta-analysis of randomized clinical trials. Gynecological Endocrinology, 2019. PubMed 30614282.
8. Fatima K, et al. Effects of myo-inositol vs. metformin on hormonal and metabolic parameters in women with PCOS: a meta-analysis. Irish Journal of Medical Science, 2023. PubMed 37148410.
9. Kelly FA, et al. Comparison of metformin with inositol versus metformin alone in women with polycystic ovary syndrome: a systematic review and meta-analysis. Endocrine, 2025. PubMed 39331347.
10. Duan M, 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. PubMed 41757236.
11. Showell MG, et al. Inositol for subfertile women with polycystic ovary syndrome. Cochrane Database of Systematic Reviews, 2018. PubMed 30570133.
12. NHS. Polyendocrine metabolic ovarian syndrome (PMOS). nhs.uk/conditions/polyendocrine-metabolic-ovarian-syndrome-pmos.