In any group of women with PMOS, inositol comes up in almost every conversation. A friend whose periods came back, a gynaecologist who suggested it, another who just shrugged. Sooner or later you wonder what those sachets are really worth.
PMOS affects about one woman in eight, and for many of them insulin resistance sits at the heart of the syndrome. That is exactly where inositol is supposed to work.
A molecule your body already makes
Inositol is neither a herb nor a drug. It is a small molecule your body makes itself from glucose. Inside the cells, it helps pass on the insulin signal, and the signal of FSH, the hormone that ripens the follicles (Greff et al., 2023). Two forms matter here, myo-inositol and D-chiro-inositol. The body turns the first into the second with an enzyme whose activity depends on insulin.
In 2011, three researchers put forward a hypothesis that explains the interest in PMOS. In the ovaries of affected women, this enzyme would be overactive, the ovary would run short of myo-inositol, and egg quality would suffer (Carlomagno et al., 2011). It is a hypothesis, not a proof. It explains why people talk about a ratio between the two forms rather than just an amount.
What the trials show
A meta-analysis published in 2018 pooled ten randomised trials, with 362 women on inositol, 179 on placebo and 60 on metformin. On inositol, the ovulation rate was 2.3 times higher than on placebo, and periods came more often. None of these trials measured live births (Pundir et al., 2018).
On the insulin side, another meta-analysis of nine trials, close to five hundred women, found lower fasting insulin and a lower insulin resistance score with myo-inositol (Unfer et al., 2017). Two of its authors worked for a pharmaceutical company.
In 2023, a team at Semmelweis University in Budapest brought together 26 trials and 1,691 women. On inositol, the chance of having regular cycles was 1.8 times higher than on placebo, testosterone and blood sugar came down a little, and inositol did no worse than metformin on most measures.
Why the international guideline stays cautious
The authors of the 2023 international guideline ran their own review, with 30 trials and 2,230 women. Their reading is more reserved. Myo-inositol or D-chiro-inositol improve some metabolic markers, D-chiro-inositol might help ovulation, and for everything else the evidence is limited and inconclusive. Myo-inositol does cause fewer stomach upsets than metformin (Fitz et al., 2024).
The guideline draws a simple position from this. Inositol can be considered according to each woman's preferences, with little harm and a possible benefit on metabolic markers, but limited clinical benefit for ovulation, excess hair and weight. It does not recommend any particular form or amount, because good-quality trials are missing (Teede et al., 2023).
The reviews reach different conclusions because they do not include the same trials, and many of those trials are small. One of the first placebo-controlled trials, published in 2007 and cited for years, was even retracted by its journal in 2023 (PubMed record).
The 40 to 1 ratio
In my PMOS guide, I give myo-inositol on its own or combined with D-chiro-inositol in a 40 to 1 ratio, with 2 to 4 g of myo-inositol a day, taken once or split in two. That ratio rests on a trial run at Sapienza University in Rome. Fifty-six women were split into seven groups of eight, each with a different blend, for three months. The 40 to 1 blend gave the best results on ovulation, and the more D-chiro-inositol was added, the more the effect faded (Nordio et al., 2019).
Eight women per group is very few. The result fits the ovary hypothesis, but it still needs confirming. If you want to try it, talk to your GP first, especially if you already take a medicine or are trying for a baby. The international guideline also asks women to tell their doctor, because the content and quality of supplements vary from one brand to another.
What the supplement cannot replace
Inositol works on the same lever as your plate, walking and muscle, which is how your cells respond to insulin. It cannot make up for meals that send your blood sugar soaring three times a day. In my guide, it comes first among the supplements together with vitamin D, but only after the chapters on food and movement.
The foundation stays the same. Protein and fat at every meal, vegetables and protein before the carbs, ten to fifteen minutes of walking after lunch and dinner, two to three strength sessions a week. I have covered the plate in PMOS and diet and the blood tests to ask for in the article on insulin resistance.
If you take inositol, write down your cycles, your waist measurement and your blood results before you start, then again after a few months. Those numbers will tell you better than any forum whether it is doing anything for you.
This article is for information only and does not replace medical advice.