A sugar alcohol that forms the backbone of intracellular signalling molecules. It was once classified as a B vitamin, then reclassified when it became clear the body synthesises it — the kidneys make several grams a day.
Inositol is unusual in this catalogue: a supplement with a coherent trial base in one specific condition. In polycystic ovary syndrome, myo-inositol at 2 g twice daily has improved ovulation rates and insulin measures across a number of randomised trials, enough that some gynaecological guidance mentions it as an option while noting the evidence quality is low to moderate. It has also been studied in gestational diabetes risk and, at much higher doses of 12–18 g/day, in panic disorder and obsessive-compulsive disorder, where the trials are decades old and very small. Outside those, the general "mood and sleep" marketing has essentially nothing behind it.
Sold as
Powder, Capsule, Myo-inositol and D-chiro-inositol 40:1 blend, Component of women's health formulas
Regulation: No Tolerable Upper Intake Level has been established. Sold as a dietary supplement in the US with no pre-market efficacy review; in some European markets it is marketed as a food for special medical purposes for polycystic ovary syndrome.
What the research used
The amounts, forms and durations published trials of Inositol (myo-inositol) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Polycystic ovary syndrome
2000–4000 mg
Moderate
Myo-inositol powder, usually 2 g twice daily · 3–6 months
The most-replicated regimen. Trials are mostly small and from Italian centres, several with manufacturer involvement, and reviews rate the certainty as low to moderate.
Studied for Panic disorder and obsessive-compulsive disorder
12000–18000 mg
Limited
Powder, divided doses · 4–6 weeks
From very small crossover trials conducted in the 1990s, typically with 20 or fewer participants. These doses cause substantial gastrointestinal upset and have not been replicated since.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.
🔀Split through the day
Limited
Trials divided the daily amount into two or more doses, mainly to manage gastrointestinal tolerance at gram quantities. No trial has compared dosing schedules.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
The evidence in depth
Best supported: Moderate
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
In polycystic ovary syndrome, myo-inositol has improved insulin resistance measures and ovulation rates across several randomised trials, with effects broadly comparable to metformin in head-to-head studies. The trials are small and mostly from a few centres, so confidence is limited by the evidence base rather than by direction of effect.
What may be going on: Inositol phosphoglycans act as second messengers in insulin signalling; the ratio of myo- to D-chiro-inositol in ovarian tissue appears altered in polycystic ovary syndrome.
Moderate evidence. Several human studies point the same way, with real caveats.
Some trials report a lower incidence of gestational diabetes with myo-inositol in women at raised risk. A Cochrane review concluded the evidence is of low quality and comes from small trials in a limited number of settings.
Limited evidence. Early human work, or mostly lab and animal research.
Very small trials from the 1990s reported reductions in panic attack frequency at 12–18 g/day. The studies are tiny and old, and the doses are far beyond what most products supply or most people would tolerate.
Limited evidence. Early human work, or mostly lab and animal research.
Systematic review · The Cochrane database of systematic reviews · 2004 · PMID 15106232
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Too much
Gastrointestinal upset at high doses
Nausea, gas, bloating and diarrhoea are common above about 12 g/day and were the main dose-limiting effect in the psychiatric trials. The 4 g/day used in polycystic ovary syndrome is usually tolerated.
Worth knowing
Polycystic ovary syndrome needs a diagnosis and a plan
Irregular cycles have several possible causes, and polycystic ovary syndrome carries metabolic and fertility implications that need proper assessment. Inositol as a self-selected substitute for that assessment is the practical risk.
Interaction
Additive effect with glucose-lowering treatment
Interacts withMetformin, insulin, sulfonylureas
Because it improves insulin sensitivity, inositol may add to metformin or other glucose-lowering drugs. Anyone using both should be alert to lower blood glucose readings.
Worth knowing
Ratio products are marketed beyond their evidence
The 40:1 myo- to D-chiro-inositol ratio is promoted as physiological, based on plasma ratios rather than on comparative trials. Most of the trial evidence used myo-inositol alone.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.