The short answer
- Safe together? No known harmful interaction. Inositol is a sugar alcohol the body also makes; side effects are mild (nausea, bloating) at usual doses.
- Typical doses: myo-inositol 2 g twice daily (4 g/day), sometimes with D-chiro-inositol in a 40:1 ratio; metformin as prescribed, often 500–2,000 mg/day.
- Evidence: the 2023 international PCOS guideline describes inositol as an experimental therapy with limited evidence; metformin has stronger evidence for metabolic outcomes.
What metformin and inositol do
Metformin lowers glucose production by the liver and improves insulin sensitivity. In PCOS it can help with insulin resistance, weight and, in some women, regularity of periods. Its main drawback is gut side effects (nausea, diarrhoea), reduced by starting low, taking it with meals or using the extended-release form. Long-term use can lower vitamin B12.
Inositol is involved in insulin signalling inside cells. Myo-inositol and D-chiro-inositol are sold as supplements, often marketed for PCOS, fertility and blood sugar. Small trials suggest myo-inositol may improve insulin resistance, ovulation and cycle regularity, with fewer side effects than metformin, but study quality is mostly low.
What the evidence says
| Question |
Current evidence |
| Does inositol improve PCOS symptoms? |
Possibly modest benefits on metabolic markers and ovulation; evidence is low certainty |
| Inositol vs metformin |
Small trials show broadly similar effects on some markers, with fewer gut side effects for inositol |
| Combination vs metformin alone |
Few small studies; no robust evidence of extra benefit |
| Inositol for fertility (IVF, ovulation induction) |
A Cochrane review (2018) found the evidence too uncertain to recommend it |
| Inositol in type 2 diabetes |
Very limited data; not a substitute for diabetes medicines |
The 2023 International Evidence-based Guideline for PCOS lists inositol (in any form) as an experimental therapy: people may choose it after discussion, but benefits on clinical outcomes are not established.
If you decide to try both
- Tell your doctor and pharmacist, so they can track effects and side effects.
- Start one at a time, so you know which causes any stomach upset.
- Choose a product with a clear dose per serving (myo-inositol in grams) and third-party testing; avoid “proprietary blends”.
- Check blood glucose if you have diabetes; the combination could, in theory, add to the glucose-lowering effect, though metformin alone rarely causes hypos.
- Don’t stop metformin or other prescribed treatment in favour of a supplement without advice.
- If you’re trying to conceive or are pregnant, discuss both with your doctor; folic acid remains essential.
More on judging supplements is in the vitamins and supplements guide, and on metformin and other diabetes medicines in the diabetes care guide.
Frequently asked questions
Can inositol replace metformin?
Not without your doctor’s agreement. For people with diabetes, metformin is a proven treatment; inositol is not. In PCOS, some women choose inositol because of metformin side effects, but evidence is weaker.
When should I take inositol with metformin?
Commonly with meals, morning and evening, alongside metformin. Taking them together doesn’t cause a known problem.
Does inositol cause low blood sugar?
Hypoglycaemia from inositol alone is not expected. People on insulin or sulfonylureas should monitor glucose when adding any supplement that may affect insulin sensitivity.
Sources and further reading
CanforaMedicine Guide
