Inositol and PCOS: what myo-inositol can and can't do, doses used in studies, and safety

What is inositol? Myo-inositol and D-chiro-inositol

Inositol is a vitamin-like compound. It isn’t a true vitamin: your body makes it, and you also get some from ordinary food.

You’ll see two forms on labels:

• myo-inositol – the main form in the body and in supplements;
• D-chiro-inositol (DCI) – added to combination products, most often in a 40:1 ratio.

Interest in it comes from insulin: inositol is thought to improve how cells respond to insulin. Insulin resistance is common in polycystic ovary syndrome (PCOS), hence the idea of a supplement.

What inositol has been studied for

Nearly all the research on inositol in women falls into three areas:

• PCOS – periods, ovulation, metabolism, excess hair growth (hirsutism) and acne;
• preparing for IVF in women with PCOS;
• pregnancy – preventing gestational diabetes.

Outside these areas – for “hormone balance” without a diagnosis, for example – there’s no reliable evidence of benefit.

Inositol and PCOS: what the 2023 guideline says

The 2023 International Evidence-based Guideline for PCOS (polycystic ovary syndrome) was developed with the European Society of Human Reproduction and Embryology (ESHRE), the Endocrine Society and the American Society for Reproductive Medicine. A systematic review was done for it: 30 trials with 2,230 women, half of them run in Italy. Its authors concluded that the evidence for inositol is limited and inconclusive, with low to very low certainty.

What the guideline itself says:

• inositol in any form can be considered, based on individual preferences – harm is limited and metabolic measures may improve, but clinical benefit for ovulation, hair growth and weight is limited;
• metformin should be considered over inositol for hirsutism and central fat, although metformin causes more stomach side effects;
• no specific type, dose or combination can be recommended yet, because of a lack of quality evidence;
• women taking inositol or other complementary therapies are encouraged to tell their health professional;
• supplements aren’t regulated like medicines, so doses and quality can vary.

So inositol isn’t a PCOS treatment. It’s an option you can try with your doctor, with modest expectations. For other women’s health topics, see supplements for women.

Where inositol fits in PCOS care

Inositol isn’t first-line in the 2023 guideline. Here’s the main framework:

Compared head to head with metformin, the 2024 review found:

• stomach side effects: probably fewer with myo-inositol;
• hirsutism and waist-to-hip ratio: metformin may work better;
• BMI: the evidence is very uncertain;
• pregnancy and ovulation: probably no difference.

Myo-inositol or 40:1: forms and doses used in studies

D-chiro-inositol comes with a specific caution. In a small 2012 study of 54 women with PCOS having IVF, egg quality and ovarian response got worse as the DCI dose went up. The journal later published an expression of concern about this paper, so treat the finding with care. Still, it’s a reason not to take high-dose DCI before IVF without medical advice.

How to take myo-inositol

1. Get a diagnosis first. Under the 2023 guideline, adults are diagnosed with PCOS if they have two of three features. These are irregular ovulation (cycles shorter than 21 or longer than 35 days, or fewer than 8 a year), excess androgens on examination or blood tests, and polycystic ovaries on a scan or a raised AMH level. Other causes, such as thyroid problems, are ruled out first.
2. Agree the dose. Studies most often used 2 g of myo-inositol twice a day, but the guideline doesn’t recommend a specific dose.
3. Count the folic acid. Many inositol products contain 200–400 mcg of folic acid. NIH ODS sets the upper limit at 1,000 mcg a day from supplements and fortified foods, so add it to what’s in your multivitamin.
4. Review it with your doctor after a few months: your cycle, blood tests and how you feel. If nothing has changed, there’s little point continuing.

What not to expect from inositol

To avoid spending months and money on false hopes, keep the guideline and review findings in mind:

• inositol doesn’t replace healthy eating and physical activity, which the guideline recommends for all women with PCOS;
• it isn’t a substitute for the combined pill if you need to regulate periods or reduce acne and hair growth;
• it doesn’t treat infertility: letrozole, prescribed by a specialist, is first-line for inducing ovulation;
• its effect on weight is very uncertain on current evidence;
• for teenagers, the guideline doesn’t recommend a specific type or dose either, so decide with a paediatric specialist.

Inositol side effects and safety

Inositol is usually well tolerated. A 2011 safety review found mild gut symptoms – nausea, wind and diarrhoea – only at the highest dose studied, 12 g of myo-inositol a day. The authors added that safety data are still scarce.

In the 2024 review, myo-inositol caused fewer stomach side effects than metformin, and those it did cause were mild and settled on their own.

Inositol in pregnancy and when trying to conceive

A 2023 Cochrane review pooled 7 small trials with 1,319 pregnant women, six of them in Italy. The pooled data suggested myo-inositol may lower the risk of gestational diabetes, but the certainty was low to very low. Doses and timing varied, and it’s unclear how far the results apply elsewhere. No side effects in mothers were found in the five trials that looked. Its authors call for larger trials before firm conclusions.

What that means for you:

• in pregnancy, take inositol only if your doctor or midwife agrees;
• if you took inositol before getting pregnant, mention it at your booking appointment;
• you still need folic acid 400 mcg – see our pregnancy vitamins guide.

Who should check with a doctor first

• You take metformin, hormonal contraception or other PCOS medicines. The guideline specifically advises telling your health professional about supplements.
• You’re preparing for IVF. Ask whether inositol makes sense and in which form, especially if the product contains D-chiro-inositol.
• You have diabetes: anything that may affect insulin sensitivity needs your glucose checked with your diabetes team.
• You’re pregnant or breastfeeding. Evidence is thin, so it’s your doctor’s call.

How to choose on iHerb

Before you add it to your multivitamin, work out how much folic acid the inositol itself brings:

1. Agree the form with your doctor: usually plain myo-inositol.
2. Check how much inositol is in one serving: a scoop for powders, often several capsules for capsules.
3. For a 40:1 blend, check the D-chiro-inositol dose.
4. Add up the folic acid from this and your other supplements.
5. Prefer products with independent quality testing (USP, NSF or similar marks): the guideline notes that supplement quality varies.

Our supplement label guide shows how to read a Supplement Facts panel. More articles are collected in our women’s health section.