Inositol
Inositol is a six-carbon sugar alcohol, and myo-inositol, the form that dominates both the body and the supplement shelf, is the backbone of the second messengers that carry signals for insulin, FSH, and several neurotransmitter receptors. The body makes several grams a day from glucose, so it is not a vitamin. Its evidence splits in two: a consistent set of PCOS trials at 4 g a day, and a few small psychiatric trials from the 1990s at doses three to four times higher than most products suggest.
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What It Does
Myo-inositol is built into phosphatidylinositol in cell membranes. When a receptor activates phospholipase C, the membrane lipid PIP2 splits into IP3, which releases calcium from intracellular stores, and diacylglycerol, which activates protein kinase C. Inositol phosphoglycans also relay part of the insulin signal. Tissues convert myo-inositol to D-chiro-inositol through an insulin-dependent epimerase, and each tissue keeps its own ratio of the two. In insulin resistance, the theory goes, high insulin drives the ovarian epimerase too hard and depletes the myo-inositol that FSH signaling depends on.
The PCOS evidence is the strongest. A 2012 systematic review and a 2017 meta-analysis by Unfer and colleagues found that myo-inositol, usually 2 g twice daily with folic acid, lowered fasting insulin and HOMA-IR, with smaller and less consistent effects on testosterone (Unfer 2012, Unfer 2017). The trials are small and short, much of the literature comes from a few Italian groups, and some authors disclose ties to inositol manufacturers. Ovulation and pregnancy outcomes are less well established than the metabolic markers. Ratios matter: among combinations compared head to head, a 40:1 myo to D-chiro ratio performed best for ovulation (Nordio 2019), and high D-chiro doses were associated with poorer oocyte quality in women undergoing IVF (Isabella 2012).
The psychiatric trials came from one Israeli group in the 1990s. Inositol at 12 g a day reduced panic attack frequency in 21 patients (Benjamin 1995) and improved depression scores in 28 (Levine 1995); 18 g a day helped obsessive-compulsive symptoms in 13 (Fux 1996). In a crossover trial of 20 patients with panic disorder, 18 g a day of inositol reduced panic attacks about as much as fluvoxamine (Palatnik 2001). All of these were small, and a 2014 meta-analysis found no significant overall effect on depression or anxiety scores (Mukai 2014). The dose gap is the practical problem: products sold for mood often provide 500 mg to 2 g, a fraction of anything tested.
Forms and Bioavailability
Absorption ratings are broad tiers. Head-to-head human trials rarely support a finer ranking among well-absorbed forms.
| Form | Absorption | Typical dose | Cost | Labels |
|---|---|---|---|---|
| Myo-Inositolinositol, myo-inositol powder | ModerateAbsorbed by sodium-dependent transporters shared with glucose; gram doses are needed because the body already makes and holds grams. | 2-4 g daily; 12-18 g in psychiatric trials | Low | 55 |
| D-Chiro-InositolDCI | ModerateMade in tissues from myo-inositol by an insulin-dependent epimerase. Supplemental DCI is absorbed but needed only in milligrams. | 50-600 mg daily, alone or with myo-inositol | High | 31 |
| Myo-Inositol and D-Chiro-Inositol (40:1)40:1 blend, Ovasitol | ModerateCombines both isomers in roughly the ratio found in plasma. Absorption is that of each component. | 2 g myo plus 50 mg DCI, twice daily | Moderate |
Labels column: number of labels named as inositol products that list this form. A product can list more than one form, and some labels do not state a form at all.
"Inositol" without a qualifier on a label means myo-inositol. Powder is the practical form: 4 g is about a level teaspoon, it dissolves in water, and it tastes mildly sweet. D-chiro-inositol is sold separately and in fixed blends. Inositol hexanicotinate, sold as "no-flush niacin," is a niacin product; it does not count as inositol and releases little usable niacin either.
Typical Dosing
PCOS trials mostly used 4 g of myo-inositol a day (2 g twice daily), often with 400 mcg folic acid. The older panic, depression, and OCD trials used 12-18 g a day.
What to Look For
For PCOS, the trial regimen is 2 g of myo-inositol twice daily, often with 200 mcg of folic acid per dose. Pre-measured sachets make that easy; capsules at 500 mg would mean eight a day. Give it three months before judging cycle regularity. A 40:1 blend is reasonable; products where D-chiro-inositol makes up a large share are not supported.
For panic or depression, the tested doses are 12-18 g a day, about four heaping teaspoons, and GI side effects are more common at that level. That is a conversation to have with the prescriber, and it is not a reason to stop a medication.
Red flags: "hormone balance" blends that hide the inositol dose, 500 mg capsules sold for PCOS, and products dominated by D-chiro-inositol. Green flags: grams of myo-inositol per serving stated plainly, a stated ratio for blends, and third-party testing.
Myo-Inositol
The form behind nearly all of the PCOS and psychiatric data, and cheap as a powder.
What to look for: Pure myo-inositol powder or 2 g sachets. Grams per serving on the panel.
Find on iHerb Affiliate linkMyo-Inositol and D-Chiro-Inositol (40:1)
Convenient for PCOS, with one trial supporting the ratio. Myo-inositol alone remains the better-studied option.
What to look for: 2 g myo-inositol and 50 mg D-chiro-inositol per serving, ratio stated.
Find on iHerb Affiliate linkD-Chiro-Inositol
Rarely needed on its own. More is not better for the ovary.
What to look for: Use only with a clinician's reason, and only in small doses alongside myo-inositol.
Find on iHerb Affiliate linkProducts on Titrate
8,650 of the 214,745 labels in the NIH Dietary Supplement Label Database list inositol as an active ingredient (3,697 on market). 389 are sold as inositol products by name.
Top on-market labels, ranked by label completeness and simplicity:
Inositol does not have its own search category yet. Each product above links to its full label in the NIH database.
Related Tools
References
Unfer V, Carlomagno G, Dante G, Facchinetti F. Effects of myo-inositol in women with PCOS: a systematic review of randomized controlled trials. Gynecol Endocrinol. 2012;28(7):509-515.
Unfer V, Facchinetti F, Orrù B, Giordani B, Nestler J. Myo-inositol effects in women with PCOS: a meta-analysis of randomized controlled trials. Endocr Connect. 2017;6(8):647-658.
Nordio M, Basciani S, Camajani E. The 40:1 myo-inositol/D-chiro-inositol plasma ratio is able to restore ovulation in PCOS patients: comparison with other ratios. Eur Rev Med Pharmacol Sci. 2019;23(12):5512-5521.
Isabella R, Raffone E. Does ovary need D-chiro-inositol? J Ovarian Res. 2012;5(1):14.
Benjamin J, Levine J, Fux M, et al. Double-blind, placebo-controlled, crossover trial of inositol treatment for panic disorder. Am J Psychiatry. 1995;152(7):1084-1086.
Levine J, Barak Y, Gonzalves M, et al. Double-blind, controlled trial of inositol treatment of depression. Am J Psychiatry. 1995;152(5):792-794.
Fux M, Levine J, Aviv A, Belmaker RH. Inositol treatment of obsessive-compulsive disorder. Am J Psychiatry. 1996;153(9):1219-1221.
Palatnik A, Frolov K, Fux M, Benjamin J. Double-blind, controlled, crossover trial of inositol versus fluvoxamine for the treatment of panic disorder. J Clin Psychopharmacol. 2001;21(3):335-339.
Mukai T, Kishi T, Matsuda Y, Iwata N. A meta-analysis of inositol for depression and anxiety disorders. Hum Psychopharmacol. 2014;29(1):55-63.
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