NOW Inositol Powder Review (2026): Best for PCOS?
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Inositol is one of the few supplements in the “metabolism” aisle that actually has a large body of randomized trials behind it — but only for one group of people. NOW Foods Inositol Powder is the best-selling version on iHerb: 4.8★ from more than 10,000 ratings, plain myo-inositol with nothing else in the tub, and about 40 cents a day at research doses. In this NOW Inositol Powder review we explain what the trials show for insulin resistance and PCOS, why the label’s ¼-teaspoon serving is far below the amount used in studies, how the powder compares with the pricier 40:1 blends, and who is wasting their money on it.
NOW Foods Inositol Powder at a Glance
| Product | NOW Foods, Inositol Powder, 8 oz (227 g); also sold in 1 lb, 4 oz and 2 oz sizes |
| Form | 100% myo-inositol powder — no fillers, sweeteners or flavors |
| Label serving | ¼ level teaspoon (730 mg) once daily, dissolved in water |
| Research dose | 2 g twice daily (4 g/day) ≈ 1½ teaspoons a day — roughly 5½ label servings |
| Servings | ~311 label servings; ~57 days at the 4 g research dose |
| Price | ~$22.04 at the time of writing (≈ 39 cents per day at 4 g); 1 lb tub ~$36.74 (≈ 32 cents per day) |
| iHerb rating | 4.8★ from 10,300+ ratings; 750+ sold in the past 30 days |
| Taste | Mildly sweet (inositol is about half as sweet as sugar), dissolves easily |
| Best for | Women with PCOS or insulin resistance who want a well-studied, low-side-effect option |
| Skip if | You don’t have PCOS or insulin resistance and are hoping for general weight loss |
| Our rating | 4.3 / 5 — excellent value and strong evidence for its intended use; label dose is too low |
What Inositol Is and How It Works
Inositol is a sugar alcohol your body makes from glucose and that you also get from beans, citrus fruit and whole grains. Two forms matter for metabolism. Myo-inositol is by far the most abundant and the one in this tub; D-chiro-inositol is made from it by an enzyme that insulin controls. Both act as “second messengers” for insulin — small molecules that carry the hormone’s signal from the cell surface to the machinery inside that takes up glucose.
In polycystic ovary syndrome, that signaling is disrupted. Cells become less responsive to insulin, the pancreas compensates by producing more of it, and high insulin pushes the ovaries to make excess androgens — which is what drives irregular cycles, acne and hair changes. Myo-inositol supplementation appears to restore part of that signaling, lowering insulin and, downstream, androgens. The mechanism matters because it tells you who benefits: people whose problem is insulin signaling. If you want the bigger picture on how insulin resistance drags down metabolism, start with our insulin resistance guide; if PCOS specifically is your situation, our PCOS and metabolism guide covers diet, training and sleep — this review is the supplement chapter that guide points to.
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What the Research Says
Insulin and HOMA-IR: consistent improvements
A 2017 meta-analysis in Endocrine Connections by Unfer and colleagues pooled nine randomized controlled trials (247 women on myo-inositol, 249 controls). Fasting insulin fell significantly (standardized mean difference −1.02, p = 0.009), as did HOMA-IR, the standard index of insulin resistance (SMD −0.585, p = 0.041). Testosterone trended down without reaching significance across all trials, while sex hormone-binding globulin — the protein that mops up free testosterone — rose significantly in the studies lasting 24 weeks or longer. The authors’ conclusion was that myo-inositol improves the metabolic profile of women with PCOS and, over time, reduces hyperandrogenism.
The largest analysis: inositol versus placebo and versus metformin
The most comprehensive work to date is a 2023 systematic review and meta-analysis in Reproductive Biology and Endocrinology by Greff and colleagues, covering 26 RCTs and 1,691 women. Against placebo, inositol increased the chance of a normalized menstrual cycle (risk ratio 1.79), lowered BMI by 0.45 kg/m², reduced total testosterone by about 20 ng/dL, raised SHBG, and lowered fasting glucose by about 3 mg/dL along with the insulin response to a glucose load. Against metformin — the prescription standard for insulin resistance in PCOS — inositol was non-inferior for cycle normalization and BMI, and it caused far fewer side effects: 7% of women reported them on inositol versus 53% on metformin.
The honest caveats
Three things temper the enthusiasm. Many of the underlying trials are small and short, and several come from a handful of research groups. The BMI effect — about half a BMI point — is real but modest; nobody should expect inositol to be a weight-loss supplement. And the 2023 international evidence-based PCOS guideline, which reviewed the same body of research, concluded that inositol could be considered for PCOS but rated the evidence as limited and still favoured metformin where the metabolic problem is significant. That is a reasonable position: inositol is a well-tolerated first step with decent evidence, not a replacement for medical care. Outside PCOS and insulin resistance, there is very little evidence that inositol changes weight or metabolism at all.
The Dosing Problem: Why the Label Serving Is Too Small
This is the single most important thing to understand before you buy. NOW’s label suggests ¼ teaspoon — 730 mg — once a day. Almost every trial in the meta-analyses above used 2 g of myo-inositol twice daily, for a total of 4 g. That is more than five label servings. Taking the label dose and expecting the trial results is like taking a fifth of a prescription and wondering why it isn’t working.
The good news is that the powder makes this easy and cheap. One level teaspoon of NOW’s inositol is roughly 2.9 g, so a heaped half-teaspoon in the morning and another in the evening gets you to about 4 g. A kitchen scale is more accurate than spoons; if you own one, weigh 2 g per dose. Dissolve it in water, coffee or a smoothie — it tastes faintly sweet and has no aftertaste. At 4 g a day the 8 oz tub lasts about eight weeks and the 1 lb tub about four months.
How long before you notice anything
Insulin markers improved in trials as short as 8–12 weeks. Cycle regularity and androgen-related changes took longer — the SHBG increase in the 2017 analysis only appeared in trials of 24 weeks or more. Plan on a three-month trial at the full dose before judging, ideally with before-and-after labs (fasting insulin, glucose, HbA1c) so you’re measuring rather than guessing.
Pure Myo-Inositol vs 40:1 Blends: Do You Need D-Chiro?
Many PCOS-branded products combine myo-inositol and D-chiro-inositol in a 40:1 ratio, matching the proportion found in blood, and charge two to four times as much. Some trials suggest the combination restores ovulation a little faster than myo-inositol alone; others show no difference; and high doses of D-chiro on its own appear to worsen egg quality, which is why the ratio matters. The pragmatic view: most of the metabolic evidence — insulin, HOMA-IR, glucose — comes from plain myo-inositol at 4 g a day, which is exactly what NOW’s powder provides at a fraction of the price. If your priority is fertility and your clinician recommends the blend, follow that advice. If your priority is insulin resistance, start with plain myo-inositol.
| NOW Inositol Powder (myo only) | Typical 40:1 blend capsules | |
|---|---|---|
| Myo-inositol per day at research dose | 4 g (you measure) | 2–4 g (fixed capsules) |
| D-chiro-inositol | None | 50–100 mg |
| Evidence for insulin/HOMA-IR | Strongest (most trials used myo alone) | Similar; fewer trials |
| Cost per day (approx.) | $0.32–0.39 | $0.80–1.50 |
| Convenience | Powder, needs measuring | Capsules, pre-dosed |
Pros and Cons
| Pros | Cons |
|---|---|
| 26-trial meta-analysis supports lower insulin, glucose and androgens in PCOS | Almost no evidence of benefit for people without PCOS or insulin resistance |
| Non-inferior to metformin for cycle regularity and BMI, with far fewer side effects | Label serving (730 mg) is about one-fifth of the research dose |
| Single ingredient, no fillers; vegan, gluten-free | Powder must be measured — a scale helps |
| Very cheap per day at full dose | Weight effect is small (about half a BMI point) |
| Pleasant mild sweetness, dissolves completely | Mild GI upset possible at very high doses (12 g+) |
Side Effects and Who Should Be Careful
Inositol is unusually well tolerated. At 4 g a day the trials report side-effect rates close to placebo; nausea, gas and loose stools appear mainly at doses of 12 g or more, which were used in older psychiatric research. Because it lowers insulin and glucose, anyone taking diabetes medication should involve their doctor to avoid stacking effects. Pregnant women have used myo-inositol in gestational-diabetes trials under supervision, but that is a conversation for your obstetrician, not a blog. And if you have PCOS, remember that inositol addresses one piece — insulin signaling — while the foundations still do the heavy lifting: our PCOS metabolism guide and our comparison of berberine versus “metabolism tonics” cover the other evidence-based options.
Who Should Buy It — and Who Shouldn’t
Worth trying if you have PCOS, prediabetes or lab-confirmed insulin resistance and want a low-risk, low-cost option with genuine trial support — especially if metformin gave you stomach trouble. It’s also a sensible thing to discuss with your doctor if you’re trying to regularize your cycle.
Skip it if you’re metabolically healthy and hoping it will speed up fat loss. The mechanism is insulin signaling; if your insulin signaling is fine, there is nothing for it to fix. Put the money toward protein and a gym membership, and grab our free Metabolism Starter Kit for the habits that move the needle for everyone.
Verdict
NOW Foods Inositol Powder earns a 4.3 out of 5 — one of the highest ratings we’ve given a supplement — because it delivers exactly the ingredient the trials used, in a form that lets you reach the trial dose for well under 50 cents a day. The deductions are for a label serving that undersells the research and for the narrow group it actually helps. If you’re in that group, this is one of the best-value, best-evidenced supplements on iHerb. If you’re not, it’s a pleasant-tasting powder that won’t do much.
Frequently Asked Questions
How much inositol should I take for PCOS or insulin resistance? The trials that showed benefit used 2 g of myo-inositol twice a day, for 4 g total. NOW’s label serving is 730 mg (¼ teaspoon), so you need roughly a heaped half-teaspoon twice daily — or, more accurately, 2 g weighed on a kitchen scale — to match the research.
Does inositol help with weight loss? Only modestly, and only in women with PCOS: a 2023 meta-analysis of 26 trials found BMI fell by about 0.45 kg/m² versus placebo. There is no good evidence it causes weight loss in people without PCOS or insulin resistance.
Is myo-inositol as good as metformin? In the 2023 meta-analysis, inositol was non-inferior to metformin for normalizing cycles and reducing BMI, and caused far fewer side effects (7% vs 53%). The international PCOS guideline still favors metformin where metabolic problems are significant, so treat inositol as an option to discuss with your doctor, not a substitute for care.
Do I need a myo-inositol and D-chiro-inositol 40:1 blend? Most of the metabolic evidence comes from plain myo-inositol at 4 g a day. The 40:1 blends may help ovulation slightly faster in some studies but cost two to four times more. Start with plain myo-inositol unless your clinician recommends the blend for fertility.
How long does inositol take to work? Insulin and glucose markers improved in 8–12-week trials; hormone and cycle changes generally took 24 weeks or longer. Commit to at least three months at the full dose, ideally with before-and-after blood work.
Related Reading
- How to Boost Your Metabolism With PCOS
- Insulin Resistance and Metabolism: What Actually Helps
- Berberine vs “Metabolism Tonics”: An Honest Breakdown
- NOW Foods Berberine Review
For general education only; not medical advice. Statements about supplements have not been evaluated by the FDA. If you have diabetes, PCOS, are pregnant or trying to conceive, or take any medication that affects blood sugar, talk to your doctor before starting inositol. Read our full Medical Disclaimer.
