Inositol is a word that collectively refers to molecules with a similar structure, a collection of nine stereoisomers. While the term 'inositol' is used commonly with dietary supplements, it usually refers to a specific stereoisomer called myo-inositol. Inositols are pseudovitamin compounds that are falsely said to belong to the B-complex family, and are found in most foods but in highest levels in whole grains and citrus fruits.
Myo-inositol shows the most promise as a dietary supplement for promoting female fertility, restoring insulin sensitivity in instances of resistance (type II diabetes and polycystic ovarian syndrome being the most well investigated), and for reducing anxiety as well. Due to the mixed benefits to insulin resistance and fertility, myo-inositol is considered a good treatment for PCOS in women.
It also holds some promise as an anti-depressant (although not as impressive as its anxiolytic and anti-panic effects) and against some other conditions associated with anxiety such as panic disorders and binge eating. It is relatively ineffective for schizophrenia and autism, and has failed in treating PTSD despite its anti-panic effects.
In part because of its benefits to fertility and PCOS, as well as the anxiolytic effects potentially helping symptoms of PMS (dysphoria and anxiety mostly), myo-inositol is sometimes referred to as a general female health supplement. At times, the anti-depressant effects associated with this supplement seem to only work in females with males having no benefit.
It is a very safe supplement to ingest, and all side-effects associated with myo-inositol are merely mild gastrointestinal distress from high doses. High doses (usually in the 12-18g range) are required for any neurological effects while lower doses (2-4g) are sufficient for fertility and insulin sensitizing effects.