Vitamins

Inositol

Also known as: Myo-inositol, Vitamin B8, Inositol powder, MI:DCI 40:1

The PCOS breakthrough B-vitamin — myo-inositol improves ovulation and insulin sensitivity with unusually solid trial data.

By IngredientWiki Editorial Team
Fact-checked by IngredientWiki Editorial Team
Updated 2026-09-26
3 min read

In short

Inositol is a sugar-alcohol-like compound — historically called vitamin B8 — that acts as a second messenger in insulin signaling, and that one fact explains its niche: the strongest evidence base in supplements for polycystic ovary syndrome (PCOS). Myo-inositol and D-chiro-inositol help cells respond to insulin, and randomized trials show inositol improves ovulation rates, menstrual regularity, and metabolic markers in women with PCOS, at gram-level doses with an excellent safety profile. Outside PCOS, smaller bodies of evidence cover anxiety and metabolic health.

Insulin second messenger, formerly known as vitamin B8
Best evidence: PCOS — ovulation and metabolic markers
Typical dose is high by supplement standards: 2–4 g daily
Two forms: myo-inositol (MI) and D-chiro-inositol (DCI), often 40:1

What is Inositol?

Inositol is the rare supplement that graduated from 'vitamin' to 'targeted therapy' on the strength of clinical trials. Its job in the body is signaling: inositol phosphates relay insulin's message inside cells, and when that relay is sluggish — as in PCOS — supplying more inositol appears to restore it. The result is a supplement with an unusually specific, well-supported use case: 2–4 grams daily improving ovulation and metabolic markers in PCOS, with a safety profile closer to food than to medication. It is the closest thing the supplement aisle has to a first-line PCOS adjunct.

Where it comes from

Natural sources

Found in fruits, beans, grains, and nuts (especially cantaloupe and citrus)
Body synthesizes it from glucose; the kidney makes grams daily
Myo-inositol is the dominant biological form

How it's made

Supplements use pure myo-inositol powder or MI + D-chiro-inositol blends (typically 40:1, mirroring ovarian physiology).

What it does in products

Benefits

PCOS: ovulation and fertility

strong evidence

Multiple randomized trials and meta-analyses find myo-inositol (2–4 g daily) improves ovulation rates, menstrual regularity, and fertility outcomes in PCOS, comparable in some studies to metformin with fewer side effects.

PCOS metabolic markers

strong evidence

Inositol improves insulin sensitivity, androgens, and lipid markers in women with PCOS — the metabolic layer of the same benefit.

Anxiety and mood

moderate evidence

Small trials suggest inositol (12–18 g daily in most studies) may reduce panic and OCD symptoms — intriguing but based on limited, older data.

Gestational diabetes prevention

moderate evidence

Some trials find inositol reduces gestational diabetes risk in at-risk pregnancies; evidence is promising but not yet definitive.

Side effects & safety

Mild nausea or loose stools (uncommon)Mostly at high gram doses; split doses and take with food.

Headache or dizziness (rare)Transient and dose-related.

Safety considerations

Myo-inositol is increasingly used under medical supervision in pregnancy for gestational diabetes prevention; discuss with your OB before use.
May add to blood-sugar-lowering effects of diabetes drugs — monitor glucose if combining.
Not for: None well established

How it's used

In supplements

PCOS studies: 2–4 g of myo-inositol daily, typically 2 g twice daily, alone or in a 40:1 MI:DCI blend. Benefits accrue over 3–6 months. Mood studies used far higher doses (12–18 g) — not for unsupervised use.

Regulatory status

Dietary supplement under DSHEA; also GRAS for food use. Not a prescription drug in the US.
GRAS (generally recognized as safe) for food use.

Why the dose is four grams, not four hundred milligrams

Inositol breaks the usual supplement-dosing intuition. Most ingredients work in milligram amounts; inositol's effective window is 2–4 grams — up to ten times a typical capsule serving — because it competes with glucose for cellular uptake and the body already holds and turns over large inositol pools. This dose reality explains the clinical pattern: under-dosed products (500 mg capsules, one daily) generate the disappointing reviews, while 2 g twice daily reproduces the trial results. If you buy inositol for PCOS, buy powder or high-count capsules and treat the gram dose as non-negotiable.

Frequently asked questions

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

Both are inositol isomers. Myo-inositol is the dominant form and the primary driver of ovarian insulin sensitivity; D-chiro-inositol is more active in specific tissues. A 40:1 MI:DCI ratio mirrors the body's own balance and is the most studied combination for PCOS.

How long does inositol take to work for PCOS?

Hormonal and ovulatory improvements typically appear at 2–3 months, with full metabolic benefits by 6 months. Consistency matters more than timing — inositol works by gradually restoring signaling.

Can men take inositol?

Yes, and some evidence supports it for metabolic health and sperm quality parameters, but the strong evidence base is PCOS-specific. For men it is a plausible metabolic adjunct, not a proven treatment.

The bottom line

Inositol is the rare supplement with a clear, well-evidenced job: at 2–4 g daily it improves ovulation, regularity, and metabolic markers in PCOS with near-food safety. Choose the studied 40:1 blend, dose in grams, and give it three months.

References

Myo-inositol for PCOS — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=myo-inositol+PCOS+randomized
Inositol and ovulation (meta-analysis). PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=inositol+ovulation+meta-analysis
Inositol and anxiety — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=inositol+anxiety+panic+randomized
Inositol compound summary. PubChem (National Library of Medicine). https://pubchem.ncbi.nlm.nih.gov/compound/892