Short answer: Both are studied mainly for PCOS. A 2026 umbrella review found inositol improved hormone and insulin measures versus placebo; by subtype, myo-inositol looked better for metabolic and fertility outcomes, and it advised caution with D-chiro-inositol alone. Most evidence was low quality.1 No large trial has compared the two directly.
| Myo-inositol | D-chiro-inositol | |
|---|---|---|
| Umbrella review view | Myo-inositol (with or without folic acid) looked better for metabolic and fertility outcomes1 | Use alone "with caution"; combinations did not consistently beat single forms1 |
| Key PCOS trial | Pregnancy trial, 464 people with PCOS, 2 g twice a day: no fewer complications than placebo2 | 44 women with PCOS, 1,200 mg a day for 6 to 8 weeks: 19 of 22 ovulated vs 6 of 22 on placebo3 |
| Gestational diabetes | No reduction in the composite outcome in the PCOS pregnancy trial2 | 276 high-risk pregnant women, 500 mg twice a day for 12 weeks: 24.8% vs 38.1% developed it4 |
| Evidence quality | No high-quality evidence; most rated low or very low1 | Same review; the classic trial is small and short3 |
A 2026 umbrella review of 13 meta-analyses of PCOS trials found inositol lowered LH and testosterone, raised SHBG, improved insulin resistance and raised ovulation rates compared with placebo or folic acid. Against metformin, most results did not differ. None of the evidence was rated high quality. By subtype, myo-inositol looked better for metabolic and fertility outcomes, and D-chiro-inositol alone was flagged for caution.1
D-chiro-inositol. In 44 women with PCOS and obesity, 1,200 mg a day for 6 to 8 weeks lowered free testosterone, blood pressure and triglycerides compared with placebo, and 19 of 22 ovulated vs 6 of 22 on placebo.3 In 276 pregnant women at high risk of gestational diabetes, 500 mg twice a day for 12 weeks lowered the rate of gestational diabetes (24.8% vs 38.1%).4
Myo-inositol. In a JAMA trial at 13 Dutch hospitals, 464 pregnant people with PCOS took 2 g of myo-inositol twice a day or placebo until delivery. The rate of gestational diabetes, preeclampsia or preterm birth was 25.0% vs 26.8%, no real difference.2
Pooled data lean toward myo-inositol for PCOS, but the evidence is mostly low quality, and the largest, best-run trial of myo-inositol in pregnancy was negative. No large trial has compared the two forms head to head. This is general information, not personal advice. More detail: the Inositol (Myo-Inositol) guide.