Indole-3-carbinol (I3C)
Also called I3C, indole-3-carbinol, 3-hydroxymethylindole.
Best grade C across 8 graded outcomes Other Last checked
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The short answer
From 13 sourcesIndole-3-carbinol (I3C) comes from cruciferous vegetables. Small trials found it shifts estrogen metabolism toward 2-hydroxyestrone, and one 27-woman trial reported regression of cervical precancer. Evidence is small, old and mostly surrogate. Health Canada limits products to 400 mg a day, rules them out in pregnancy and breastfeeding, and flags liver symptoms.
Every graded outcome on this page
8 rows · line = participants- CGrade CUrinary estrogen-metabolite ratioEstrogen metabolismn 60
- DGrade DJuvenile recurrent respiratory papillomatosisPapillomasn 87
- DGrade DUrinary estrogen-metabolite ratio by doseEstrogen metabolismn 57
- DGrade DRecurrent respiratory papillomatosisPapillomasn 33
- DGrade DRegression of cervical precancer (CIN II-III)Cervical cellsn 27
- DGrade DLupus disease activityLupusn 17
- DGrade DSex hormone levels and tolerabilityHormonesn 17
- DGrade DVulvar precancer (VIN) symptoms and lesionsVulvar cellsn 12
- 5 more graded outcomes, in the evidence chapter
Key facts
- What it is
- An indole (C9H9NO) sourced from Brassica vegetables or made synthetically; after oral doses, DIM is the only I3C product detected in blood1017
- Top studied outcomes
- Urinary estrogen-metabolite ratio up (C to D); cervical precancer regression in one small trial (D)289
- Studied dose range
- 50 to 400 mg/day in controlled trials; up to 800 mg/day for 4 weeks in a phase I study489
- Health Canada limit
- 200 to 400 mg/day for estrogen-metabolism products; not over 400 mg/day1
- Liver enzyme effect
- At 800 mg/day, CYP1A2 activity rose about 4.1-fold in 94% of women in a phase I study4
- Vulvar precancer
- A 2025 meta-analysis of treatments for usual-type VIN judged indole-3-carbinol ineffective12
What it is
Indole-3-carbinol (I3C) is a compound from cruciferous vegetables such as broccoli, cabbage and Brussels sprouts, sold as a supplement.
How it works
ProposedForms
Form
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Indole-3-carbinol (isolate or synthetic) | Varies101Not applicable (a single organic compound, C9H9NO) | I3C was not detectable in plasma; its product DIM peaked about 2 hours after a dose, with mean Cmax rising from 61 ng/mL at 400 mg to 607 ng/mL at 1,000 mg and no further rise at 1,200 mg.7 | 400 and 800 mg/day were tolerated in a phase I study, and no significant toxicity was seen at up to 400 mg/day for 4 weeks.48 | Estrogen metabolism, cervical and vulvar precancer, respiratory papillomas |
What the human research says
Human trials only, one row per outcome. The form line shows what each trial actually tested. Grades follow our evidence-grading policy.
Trial map
8 graded outcomes, largest study first8 graded outcomes: 5 improved, 1 mixed, 2 no effect. The largest, a 2011 open-label trial of 87 people on juvenile recurrent respiratory papillomatosis, found an improvement.
1Juvenile recurrent respiratory papillomatosis
Improved · Grade D · Open-label trial, 87 people, 2011
Children aged 2–15 with recurrent respiratory papillomatosis after repeated surgery (Russia)
Dose, result and source| Outcome | Found | People | Study | Who was studied |
|---|---|---|---|---|
| Juvenile recurrent respiratory papillomatosis | Improved | 87 | Grade D · Open-label trial, 2011 | Children aged 2–15 with recurrent respiratory papillomatosis after repeated surgery (Russia) |
| Urinary estrogen-metabolite ratio | Improved | 60 | Grade C · RCT, 1994 | Women (three arms of 20) |
| Urinary estrogen-metabolite ratio by dose | Improved | 57 | Grade D · RCT, 1997 | Women at increased risk of breast cancer, mean age 47 |
| Recurrent respiratory papillomatosis | Improved | 33 | Grade D · Open-label trial, 2004 | Adults and children with recurrent respiratory papillomatosis followed a mean 4.8 years after surgery |
| Regression of cervical precancer (CIN II-III) | Improved | 27 | Grade D · RCT, 2000 | Women with biopsy-proven CIN II-III (30 randomised, 27 analysed) |
| Sex hormone levels and tolerability | No effect | 17 | Grade D · Open-label trial, 2005 | Women from a high-risk breast cancer cohort (16 premenopausal) |
| Lupus disease activity | No effect | 17 | Grade D · Open-label trial, 2001 | Premenopausal women with probable or definite SLE |
| Vulvar precancer (VIN) symptoms and lesions | Mixed | 12 | Grade D · RCT, 2006 | Women with high-grade vulvar intraepithelial neoplasia (12 analysable) |
Every graded row
8 rowsShowing 8 of 8 outcomes. Human trials only.
CGrade CUrinary estrogen-metabolite ratioEstrogen metabolismRatio raisedRCT
n 60 · 1994
Improved The urinary 2-hydroxyestrone:estriol ratio rose substantially by month 1 and stayed up for 3 months with I3C, but not with fibre or placebo; 3 of 20 women on I3C showed no change.
- Design
- RCT, n = 60, 1994
- Population
- Women (three arms of 20)
- Form
- Indole-3-carbinol
- Dose
- 400 mg/day (vs 20 g/day cellulose fibre or placebo)
- Duration
- 3 months
- Source
- Reference 9 · PubMed 7827590 (opens PubMed)
DGrade DJuvenile recurrent respiratory papillomatosisPapillomasAbout 70% respondedOpen-label trial
n 87 · 2011
Improved 28.7% had stable remission and 41.1% had 1.5- to 10-fold longer intervals between relapses; 29.9% had no clear effect. No adverse effects were recorded.
- Design
- Open-label trial, n = 87, 2011
- Population
- Children aged 2–15 with recurrent respiratory papillomatosis after repeated surgery (Russia)
- Form
- Indole-3-carbinol
- Dose
- Not stated in the abstract
- Duration
- 12 weeks to 2 years of use (mean 8.9 months); follow-up 2 to 6.4 years
- Source
- Reference 11 · PubMed 22334925 (opens PubMed)
DGrade DUrinary estrogen-metabolite ratio by doseEstrogen metabolismRatio raised at 300 mg or moreRCT
n 57 · 1997
Improved The 2-hydroxyestrone:16-alpha-hydroxyestrone ratio rose more at 300 to 400 mg than with placebo or 50 to 200 mg (difference 0.48; 95% CI 0.30 to 0.67 at the median baseline ratio). No significant toxicity.
- Design
- RCT, n = 57, 1997
- Population
- Women at increased risk of breast cancer, mean age 47
- Form
- Indole-3-carbinol
- Dose
- 50, 100, 200, 300 or 400 mg/day, or placebo
- Duration
- 4 weeks
- Source
- Reference 8 · PubMed 9589355 (opens PubMed)
DGrade DRecurrent respiratory papillomatosisPapillomasAbout two-thirds respondedOpen-label trial
n 33 · 2004
Improved 33% had remission without further surgery, 30% had slower regrowth and 36% had no response; 12 more patients were lost to follow-up. No side effects attributed to I3C were found.
- Design
- Open-label trial, n = 33, 2004
- Population
- Adults and children with recurrent respiratory papillomatosis followed a mean 4.8 years after surgery
- Form
- Indole-3-carbinol
- Dose
- 200 mg twice daily (adults)
- Duration
- Mean 4.8 years follow-up
- Source
- Reference 5 · PubMed 15193659 (opens PubMed)
DGrade DRegression of cervical precancer (CIN II-III)Cervical cellsMore regression than placeboRCT
n 27 · 2000
Improved Complete regression on biopsy in 0 of 10 on placebo, 4 of 8 on 200 mg and 4 of 9 on 400 mg (RR 0.50, 95% CI 0.25 to 0.99; and 0.55, 0.31 to 0.99).
- Design
- RCT, n = 27, 2000
- Population
- Women with biopsy-proven CIN II-III (30 randomised, 27 analysed)
- Form
- Indole-3-carbinol
- Dose
- 200 or 400 mg/day (vs placebo)
- Duration
- 12 weeks
- Source
- Reference 2 · PubMed 10926790 (opens PubMed)
DGrade DLupus disease activityLupusNo change in disease activityOpen-label trial
n 17 · 2001
No effect The 2:16-alpha-hydroxyestrone ratio rose from 1.84 to 3.15 (p = 0.0001), but disease activity scores did not change significantly.
- Design
- Open-label trial, n = 17, 2001
- Population
- Premenopausal women with probable or definite SLE
- Form
- Indole-3-carbinol
- Dose
- 375 mg/day
- Duration
- 1 week (12 continued for 3 months)
- Source
- Reference 6 · PubMed 11789487 (opens PubMed)
DGrade DSex hormone levels and tolerabilityHormonesNo change in hormone levelsOpen-label trial
n 17 · 2005
No effect Both doses were well tolerated. Estradiol, progesterone, LH, FSH and SHBG did not change. CYP1A2 rose 4.1-fold and the estrogen-metabolite ratio 66%, with no further rise from 400 to 800 mg.
- Design
- Open-label trial, n = 17, 2005
- Population
- Women from a high-risk breast cancer cohort (16 premenopausal)
- Form
- Indole-3-carbinol
- Dose
- 400 mg/day for 4 weeks, then 800 mg/day for 4 weeks (after placebo run-in)
- Duration
- 8 weeks
- Source
- Reference 4 · PubMed 16103443 (opens PubMed)
DGrade DVulvar precancer (VIN) symptoms and lesionsVulvar cellsSymptoms better; histology unchangedRCT
n 12 · 2006
Mixed Itch (p = 0.018), pain (p = 0.028) and lesion size (p = 0.005) improved, but biopsy grade did not (p = 0.317); 200 and 400 mg did not differ.
- Design
- RCT, n = 12, 2006
- Population
- Women with high-grade vulvar intraepithelial neoplasia (12 analysable)
- Form
- Indole-3-carbinol
- Dose
- 200 or 400 mg/day (randomised dose comparison; no placebo)
- Duration
- 6 months
- Source
- Reference 3 · PubMed 16681761 (opens PubMed)
A non-randomised cancer study
1 noteHuman data that cannot be graded as an outcome here.
- Advanced ovarian cancer (2018)
- 284 women chose their own treatment arm (randomisation was abandoned). Arms that added 200 mg I3C twice daily, mostly together with EGCG, to surgery and chemotherapy reported longer median survival (60 vs 44–46 months) than chemotherapy and surgery alone. Because patients picked their arm and most I3C arms also had EGCG, the study cannot show what I3C itself does; the authors call it hypothesis-generating.13
What the numbers add up to
Each point is worked out from the data and the sources numbered on this page, with the arithmetic shown under it. It adds no new data and is not medical advice.
Mostly a biomarker story
A strong effect on a liver enzyme
Studied doses
Doses used in research, shown against the official limits. They describe the studies, not a recommendation for you.
Studied doses in research, not advice. These are amounts used in research, not personal advice. Your clinician or pharmacist can say what fits you.
Doses studied in human trials
When trials measured an effect
1 outcome| Outcome | Measured at | Basis |
|---|---|---|
| Urinary estrogen-metabolite ratio (400 mg/day) | By month 1, sustained through month 3 | 1994 trial of 60 women; ratio measured monthly9 |
Interactions and safety
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
What Indole-3-carbinol meets
2 cited rows 4 tested togetherMedicines 2
Supplements and foods 2
- Measured in a human study
- Reference summary
- Tested together in a graded trial
Choose a medicine or supplement above to read its cited rows here.
Also cited: 4 tested together
Drugs and other substances cleared by CYP1A2
PK studyMeasured in a human study
Drugs and other substances cleared by CYP1A2, for example caffeine (the probe used in the study): At 800 mg/day, I3C raised CYP1A2 activity in 94% of women (mean 4.1-fold), so substances cleared by CYP1A2 may leave the body faster. It also lowered NAT-2 activity in women whose activity was high.
Source 4, Cancer Epidemiol Biomarkers Prev (2005)
Medications and other natural health products
NIH fact sheetReference summary
Medications and other natural health products: Health Canada advises asking a health care practitioner before using products with 20 mg or more of I3C a day if you take medications or other natural health products.
Source 1, Health Canada, Natural and Non-prescription Health Products Directorate (2026)
Calcium D-GlucarateCalcium D-Glucarate page
Tested together
Urinary estrogen metabolites (2-OHE and the 2:16α-OHE ratio) with a multi-ingredient formula. Graded on the Calcium D-Glucarate page; tested together is not a claim that they work better together.
ChrysinChrysin page
Tested together
Testosterone, estrogens and strength gains (blend with 625 mg chrysin). Graded on the Chrysin page; tested together is not a claim that they work better together.
Tested together
Estrogen and DHT formation (androstenedione blend with 625 mg chrysin). Graded on the Chrysin page; tested together is not a claim that they work better together.
Tested together
Estrogen and DHT formation (androstenediol blend with 300 mg chrysin). Graded on the Chrysin page; tested together is not a claim that they work better together.
Interactions
2 agents| Agent | What the source says | Evidence |
|---|---|---|
| Medicines2 | ||
| Drugs and other substances cleared by CYP1A2e.g. caffeine (the probe used in the study) | At 800 mg/day, I3C raised CYP1A2 activity in 94% of women (mean 4.1-fold), so substances cleared by CYP1A2 may leave the body faster. It also lowered NAT-2 activity in women whose activity was high.4 | PK study |
| Medications and other natural health products | Health Canada advises asking a health care practitioner before using products with 20 mg or more of I3C a day if you take medications or other natural health products.1 | NIH fact sheet |
Check with what you take
Kept on this device onlyAdd what you take. We list every interaction our evidence profiles cite between Indole-3-carbinol and your picks, and between your picks.
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Add a supplement or medicine you take to check it against Indole-3-carbinol.
Starting from one medicine? The interaction checker rates how serious each pair is, on a curated list of 19 entries for 11 medicines. This cabinet has no severity rating: it lists every interaction row our supplement pages cite.
Who should be careful
6 groups- Pregnancy and breastfeeding
- Health Canada says not to use indole-3-carbinol products if you are pregnant or breastfeeding.1
- Trying to conceive
- Health Canada advises asking a health care practitioner before use if you are attempting to conceive.1
- Liver problems
- Ask a health care practitioner before use if you have a liver disorder, and stop and seek advice if liver symptoms appear (yellow eyes or skin, dark urine, abdominal pain), per Health Canada.1
- Symptoms of low estrogen
- Health Canada advises checking first if you have joint pain, mood or libido changes, hot flashes, night sweats, vaginal dryness or irregular periods, and stopping if such symptoms develop.1
- Hormone concerns
- For estrogen-balance products, Health Canada advises seeing a practitioner first to exclude a serious cause of hormonal imbalance.1
- Children
- In an uncontrolled Russian series, children aged 2–15 with recurrent respiratory papillomatosis took I3C for 12 weeks to 2 years with no adverse effects recorded; there are no data in healthy children.11
Reading the label, and the cost per dose
Compare any product you already have by the amount it delivers, not by capsule count or front-of-pack weight.
How to read the label
2 checksStay within 400 mg a day
I3C is not DIM
DIM is sold separately. After I3C doses, DIM was the only I3C product found in blood, but the two have separate trial records.7
Have the bottle? Photograph its Supplement Facts panel and see each line, indole-3-carbinol (i3c) included, beside the doses and forms the trials used in Label X-Ray.
Cost per studied dose, without the sales pitch
Your numbers, no product dataEnter the price, servings and the amount per serving from your label.
Where to buy Indole-3-carbinol (I3C)
Products from brands we partner with that contain Indole-3-carbinol (I3C). Check the amount per serving against the doses studied above, and the label for other ingredients.
Indole-3-carbinol (I3C) products from partner brands
1 product Paid links · we earn a commission- HerbsProNOW Foods Indole-3-Carbinol (I3C), 200 mg, 60 veg capsules
1 capsule: indole-3-carbinol 200 mg plus flax seed lignan extract 200 mg (the trials used I3C alone)
$16.74one-timeChecked Oct 15, 2026
Frequently asked questions
Short answers to common questions; the numbers link to the sources below.
Common questions
6 answersDoes I3C balance estrogen?
Does I3C help cervical or vulvar precancer?
Is I3C safe for long-term use?
Doses up to 800 mg a day were tolerated for 4 weeks, and two uncontrolled series in respiratory papillomas, with use of up to 2 years and follow-up of several years, found no side effects attributed to it. Health Canada caps products at 400 mg a day, rules them out in pregnancy and breastfeeding, and flags liver symptoms.45111
Does I3C interact with medications?
What is the difference between I3C and DIM?
I3C is the compound from cruciferous vegetables; after oral I3C, DIM was the only I3C product detected in blood. Both are sold as separate supplements.7
Sources
13 sources: trial abstracts, NIH fact sheets and reference databases. Numbers in the text link here.
Sources cited on this page
13 sources- 1Indole-3-Carbinol (Natural Health Product monograph, May 29, 2026)Guideline Health Canada, Natural and Non-prescription Health Products Directorate, 2026 · webprod.hc-sc.gc.ca
- 2Placebo-controlled trial of indole-3-carbinol in the treatment of CIN.RCT Gynecol Oncol, 2000 · PMID 10926790
- 3A randomized phase II trial of indole-3-carbinol in the treatment of vulvar intraepithelial neoplasia.RCT Int J Gynecol Cancer, 2006 · PMID 16681761
- 4A phase I study of indole-3-carbinol in women: tolerability and effects.Open-label trial Cancer Epidemiol Biomarkers Prev, 2005 · PMID 16103443
- 5Indole-3-carbinol for recurrent respiratory papillomatosis: long-term results.Open-label trial J Voice, 2004 · PMID 15193659
- 6Indole-3-carbinol in women with SLE: effect on estrogen metabolism and disease activity.Open-label trial Lupus, 2001 · PMID 11789487
- 7Single-dose and multiple-dose administration of indole-3-carbinol to women: pharmacokinetics based on 3,3'-diindolylmethane.PK study Cancer Epidemiol Biomarkers Prev, 2006 · PMID 17164373
- 8Dose-ranging study of indole-3-carbinol for breast cancer prevention.RCT J Cell Biochem Suppl, 1997 · PMID 9589355
- 9Long-term responses of women to indole-3-carbinol or a high fiber diet.RCT Cancer Epidemiol Biomarkers Prev, 1994 · PMID 7827590
- 10Indole-3-carbinol (CID 3712): C9H9NO, MW 147.17Database PubChem, NCBI, 2026 · pubchem.ncbi.nlm.nih.gov
- 11[The results of adjuvant therapy of juvenile recurring respiratory papillomatosis with the use of indole-3-carbinol].Open-label trial Vestn Otorinolaringol, 2011 · PMID 22334925
- 12Treatment Interventions for Usual-Type Vulvar Intraepithelial Neoplasia: A Systematic Review and Meta-analysis.Meta-analysis J Low Genit Tract Dis, 2025 · PMID 40476854
- 13A new promising way of maintenance therapy in advanced ovarian cancer: a comparative clinical study.Observational BMC Cancer, 2018 · PMID 30236079
What changed
Every dated change to this page: what changed, who made it and why.
Change log
2 entries| Date | Change | Who and why |
|---|---|---|
| all | Audit: added a graded row for juvenile recurrent respiratory papillomatosis (2011 uncontrolled series, n = 87) and a key fact and row note from a 2025 meta-analysis judging I3C ineffective for vulvar precancer; a CYP1A2/NAT-2 interaction row from the phase I study; a children note; a research note on a non-randomised 2018 ovarian cancer study (not graded); time-to-effect row; cost per studied dose; FAQ on cervical and vulvar precancer and long-term use; goal hub and EGCG links. Checked LiverTox, LactMed, NCCIH and MedlinePlus: no indole-3-carbinol entry (LiverTox mentions it only inside a Tribulus multi-ingredient study). PubMed searched through October 2026: no randomised I3C-alone trial newer than 2006; recent DIM trials belong on the DIM page and a 2023 multi-ingredient (EstroSense) trial was not added. | Writer: Kymata Health EditorialTrigger: A–Z audit against docs/supplements/AUDIT-STANDARD.md |
| all | Initial editorial evidence review. Every PMID checked against NCBI E-utilities esummary (title, year, journal; none retracted) and each cited abstract read; every number comes from the abstracts. None of the cited abstracts names a funder. Safety rows come only from Health Canada's Indole-3-Carbinol monograph (May 29, 2026); the CYP1A2 finding is reported as a study result, not a safety rule. Where to buy: NOW Foods I3C 200 mg (label checked 2026-10-15; contains flax lignan extract too). | Writer: Kymata Health EditorialTrigger: new page |
Supplement & Nutrient Encyclopedia. Educational information only; it is not medical advice and does not replace a clinician. Dietary supplements are not intended to diagnose, treat, cure or prevent any disease. Statements about supplements have not been evaluated by the FDA.