Inositol is a naturally occurring sugar-like compound involved in how cells respond to insulin. The most studied forms are myo-inositol and D-chiro-inositol, often combined in supplements.
It is best known as a supplement for PCOS, where some studies suggest it may support insulin sensitivity, menstrual regularity and ovulation — though the evidence is still developing and not a substitute for medical care. It is generally well tolerated. See myo-inositol benefits and inositol for PCOS.