Inositol vs Metformin for PCOS: What the Head-to-Head Trials Show
Inositol is often sold as the natural alternative to metformin for PCOS. Pooled trials say it holds its own on regular periods with fewer stomach side effects, but the 2023 international guideline still favours metformin for several goals. Here is what the comparisons found, who funded them, and who should be careful.
By Kymata Health Editorial Published Last checked
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Key takeaways
From 5 sources- A 2023 meta-analysis of 26 trials (1,691 women) found inositol made regular periods 1.79 times as likely as placebo and was non-inferior to metformin on that outcome1.
- The review that informed the 2023 international PCOS guideline was more cautious and called the evidence for inositol limited and inconclusive2.
- Metformin may do more for hirsutism and waist-hip ratio, while myo-inositol likely causes fewer gastrointestinal side effects2.
- The guideline says metformin should be considered over inositol for hirsutism and central adiposity, and that no specific inositol type, dose or combination can yet be recommended3.
- Metformin use may be linked to low vitamin B12, and the guideline suggests considering monitoring in people with risk factors3.
What this article covers
1 evidence profileThe evidence, graded
Pulled from the trial tables in our evidence profiles. Grades follow our evidence-grading policy.
Graded findings
2 rows- BGrade BRegular menstrual periods in PCOSImproved Regular periods were 1.79 times as likely as with placebo (95% CI 1.13–2.85), and inositol was non-inferior to metformin on this outcome.Inositol · Meta-analysis · n=1,691 · 2023 · PMID 36703143
- CGrade CFasting insulin and HOMA insulin resistance in PCOSImproved Fasting insulin fell (SMD −1.02) and HOMA index fell (SMD −0.59) vs control; testosterone showed only a non-significant trend down. SHBG rose only in trials of at least 24 weeks.Inositol · Meta-analysis · n=496 · 2017 · PMID 29042448
Where these trials agree: Inositol and pcos, counted row by row in the Consensus finder.
What the pooled comparisons found
Both are used because insulin resistance is common in PCOS. Metformin is a prescription insulin sensitizer; inositol, mostly myo-inositol and sometimes D-chiro-inositol, is sold as a supplement2.
The largest pooled analysis, published in 2023, combined 26 randomized trials with 1,691 women: 806 took inositol, 311 placebo and 509 metformin1. Regular cycles were 1.79 times as likely on inositol as on placebo (95% CI 1.13 to 2.85), and inositol was non-inferior to metformin for cycle regularity1.
Against placebo, inositol also lowered BMI, free and total testosterone, androstenedione and fasting glucose more, and raised SHBG1. The authors concluded inositol was non-inferior to metformin in most outcomes1.
What the 2023 international guideline concluded
A separate review written to inform the 2023 international PCOS guideline looked at 30 trials (2,230 women) and pooled 19 of them2. It found possible benefits of myo-inositol or D-chiro-inositol for some metabolic measures and of D-chiro-inositol for ovulation, and likely no difference from metformin on reproductive outcomes2. Metformin may improve waist-hip ratio and hirsutism more than inositol, and the evidence on BMI was very uncertain2.
The guideline says inositol in any form could be considered based on individual preferences and values, noting limited harm and possible metabolic benefit but limited clinical benefit for ovulation, hirsutism or weight3. It says metformin should be considered over inositol for hirsutism and central adiposity, while noting that metformin causes more gastrointestinal side effects3. Both statements rest on very low certainty evidence3.
For metformin itself, the guideline says it should be considered in adults with PCOS and a BMI of 25 or more for weight-related and metabolic outcomes, and may be considered below that BMI with limited evidence3. Our birth control vs metformin guide covers the other main option.
Side effects: where inositol has the edge
Stomach side effects are a common reason people look for an alternative to metformin2. The guideline review judged that myo-inositol likely causes fewer of them, although metformin's are typically mild and self-limited2.
The guideline adds that metformin side effects are generally dose dependent, that starting low with 500 mg increases every 1 to 2 weeks or using extended-release tablets may help, and that the suggested maximum is 2.5 g a day for adults3.
Product quality is another difference. The guideline notes that the regulatory status and quality control of inositol supplements can differ from medicines, and that doses and quality may vary between products3.
Who funded the evidence
The 2023 meta-analysis was supported by Hungary's National Research, Development and Innovation Office, and its authors declared no competing interests1. The guideline review and the guideline were primarily funded by Australia's National Health and Medical Research Council, with partnership funding from medical societies for the guideline23.
An earlier 2017 meta-analysis of 9 trials (496 women) found myo-inositol lowered fasting insulin (SMD -1.02) and the HOMA insulin-resistance index (SMD -0.59)4. Two of its authors were employees of the inositol maker Lo.Li. Pharma and a third was paid by the company as a consultant for the analysis4.
For the wider inositol evidence in PCOS, see inositol for PCOS and the inositol page.
Who should be careful
No drug interactions with inositol are well documented, but they have not been studied systematically. Keep the prescriber in the loop if any of these apply:
- Anyone already on metformin: a supplement is not a swap for a prescribed drug, and the guideline encourages people taking inositol to tell their healthcare professional3.
- Metformin and vitamin B12: metformin use may be linked to low B12, especially with diabetes, past bariatric surgery, pernicious anaemia or a vegan diet, where monitoring should be considered3. See our B12 and metformin explainer.
- Glucose-lowering medicines: inositol lowered glucose and insulin measures in trials1, so adding it to drugs that lower blood sugar is worth discussing with the prescriber.
- Pregnancy or trying to conceive: inositol trials in pregnancy were small, so use belongs in a plan with the obstetric team; the inositol page covers that evidence.
- High doses: in a safety review, mild nausea, gas and diarrhoea appeared only at the highest studied dose of 12 g a day5. One of its two authors, Vittorio Unfer, is listed as a Lo.Li. Pharma employee in the 2017 meta-analysis4.
Quick answers
Common questions
3 answersCan inositol replace metformin for PCOS?
Does inositol have fewer side effects than metformin?
On current evidence, yes: the guideline review judged myo-inositol likely causes fewer gastrointestinal side effects, though metformin's are usually mild and self-limited2.
What dose of inositol does the guideline suggest?
None. The guideline says specific types, doses or combinations of inositol cannot currently be recommended because of a lack of quality evidence3.
Keep reading
Full evidence profiles
Every graded study, studied doses, interactions and safetyRelated articles
4 articles- ArticleInositol for PCOS: More Regular Periods in 26 Trials, With Real CaveatsDoes inositol help PCOS?
- ArticleBerberine vs Inositol for PCOS: More Trials on One Side, Integrity Questions on the OtherIs berberine or inositol better for PCOS?
- ArticleAkkermansia and “GLP-1 Probiotics”: Small Trials, Modest Results and Mostly Company-Run DataDo Akkermansia probiotics boost GLP-1 or help with weight loss?
- ArticleAlpha-GPC vs Choline Bitartrate: Blood Choline Rose the Same, but One Detail DifferedIs alpha-GPC better than choline bitartrate?
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2 capsules: 900 mg myo-inositol; 60 servings
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References
5 sources. Numbers in the text link here.
Sources cited in this article
5 sources- 1Inositol is an effective and safe treatment in polycystic ovary syndrome: a systematic review and meta-analysis of randomized controlled trialsReprod Biol Endocrinol, 2023 · PMID 36703143
- 2Inositol for Polycystic Ovary Syndrome: A Systematic Review and Meta-analysis to Inform the 2023 Update of the International Evidence-based PCOS GuidelinesJ Clin Endocrinol Metab, 2024 · PMID 38163998
- 3Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary SyndromeJ Clin Endocrinol Metab, 2023 · PMID 37580314
- 4Myo-inositol effects in women with PCOS: a meta-analysis of randomized controlled trialsEndocr Connect, 2017 · PMID 29042448
- 5Inositol safety: clinical evidencesEur Rev Med Pharmacol Sci, 2011 · PMID 21845803
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