Metformin and inositol are often mentioned in the same breath and they do not sit in the same evidence bracket. The 2023 International Evidence-based Guideline recommends metformin in adults with PCOS and a BMI of 25 kg/m2 or above, primarily for metabolic features such as glucose and lipids, and states it may be considered at a lower BMI with limited evidence. Inositol is handled far more cautiously, and the reason is worth reading.

What metformin does and does not do

Metformin lowers hepatic glucose output and improves insulin sensitivity. In PCOS this translates into modest improvements in glucose and lipid measures, and in some women a return of more regular cycles. It is not a weight loss drug in any meaningful sense and it is not first line for ovulation induction, where letrozole is. Metformin is not FDA approved for PCOS; the label covers type 2 diabetes only, so its use here is off-label and well established.

What the evidence on inositol actually says

Inositol is contested, and the people contesting it include the guideline authors themselves. The systematic review and meta-analysis commissioned to inform the 2023 update concluded: "The evidence supporting the use of inositol in the management of PCOS is limited and inconclusive." Certainty of evidence across most comparisons was low to very low. The guideline reflects that, stating inositol "could be considered" based on individual preferences and values, noting limited harm, some potential for improvement in metabolic measures, yet limited clinical benefits. That is a long way from the marketing.

Side effects and monitoring on metformin

Gastrointestinal side effects are the main reason people stop. In the FDA label for immediate release metformin, diarrhoea occurred in 53.2 per cent and nausea or vomiting in 25.5 per cent of patients on monotherapy, against 9.6 per cent and 6.5 per cent for the extended release form. Metformin is contraindicated at an eGFR below 30 mL/min/1.73 m2 and carries a boxed warning for lactic acidosis. About 7 per cent of patients in trials developed subnormal vitamin B12 levels, so B12 is worth checking periodically.

	Metformin	Inositol
Guideline position	Recommended for metabolic features, BMI 25 or above	Could be considered, limited clinical benefit
Evidence certainty	Moderate	Low to very low
Regulation	FDA approved drug, off-label for PCOS	Dietary supplement, not FDA reviewed for efficacy
Main downside	Gastrointestinal effects, B12 fall	Cost, and evidence that may not hold up

If you want to try inositol, the honest framing is a low-harm experiment with uncertain benefit, run for three months against a defined outcome you can actually measure, such as cycle length. If your priority is glucose, lipids or blood pressure, metformin has the better evidence and should be discussed first.

Metformin titration per the 2023 guideline: start low, increase by 500 mg every one to two weeks, use extended release preparations to reduce side effects, with a suggested maximum daily dose of 2.5 g in adults and 2 g in adolescents. Combination with a combined oral contraceptive offers little additional benefit at BMI 30 kg/m2 or below, but may help in higher metabolic risk groups including BMI above 30, impaired glucose tolerance or high risk ethnic groups.

FDA label limits: contraindicated at eGFR below 30 mL/min/1.73 m2, initiation not recommended at eGFR 30 to 45, boxed warning for metformin-associated lactic acidosis, and annual haematological monitoring advised given the roughly 7 per cent rate of subnormal vitamin B12 in trials. Inositol trials informing the 2023 guideline compared myo-inositol and D-chiro-inositol against placebo and against metformin, with heterogeneous dosing and small samples.

Do metformin or inositol help PCOS?

What metformin does and does not do

What the evidence on inositol actually says

Side effects and monitoring on metformin

Sources

What metformin does and does not do

What the evidence on inositol actually says

Side effects and monitoring on metformin

Sources

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