Indole-3-Carbinol
Indole-3-carbinol (I3C), from Brassica species
The parent compound of DIM from cruciferous vegetables, I3C has the longest clinical track record for HPV-related cervical and laryngeal lesions but is unstable in the stomach and has largely been replaced by DIM in supplements.
At a Glance
- Best known for
- Shifts estrogen metabolism toward 2-hydroxyestrone, Studied for cervical dysplasia (CIN) regression, Studied for recurrent respiratory papillomatosis (HPV)
- Evidence
- Preliminary
- Typical dose
- 200–300 mg daily
- Type
- Compound
What is Indole-3-Carbinol?
Indole-3-carbinol (I3C) is produced when glucobrassicin, a glucosinolate in cruciferous vegetables, is broken down by the enzyme myrosinase during chopping or chewing. I3C is unstable: in the acid of the stomach it condenses into a mix of compounds, the main one being DIM, plus smaller amounts of ICZ and other oligomers. For this reason I3C is sometimes described as a 'pro-drug' for DIM. I3C was the first cruciferous compound studied in people for hormone-related conditions in the 1990s, and it remains the form used in the only randomized trials of HPV-related cervical and laryngeal lesions.
Known Health Benefits
How It Works
I3C and its condensation products activate the aryl hydrocarbon receptor, inducing CYP1A1 and CYP1A2 in the liver, which pushes estrogen metabolism toward 2-hydroxylation rather than 16α-hydroxylation. This shifts the balance away from the more proliferative 16α-hydroxyestrone, which is relevant because HPV-infected cervical cells use 16α-hydroxyestrone to drive growth. I3C also inhibits CDK6 and induces cell cycle arrest in cancer cell lines, activates phase II enzymes like glutathione S-transferase, and in animal models reduces chemically induced tumors in breast, liver, and colon. Because I3C generates a less predictable mixture of metabolites than DIM — including ICZ, which is structurally similar to dioxin in its receptor activity — some researchers consider DIM the cleaner compound.
What Research Says
A small but notable 2000 randomized, placebo-controlled trial of 30 women with CIN 2–3 found that 4 of 8 women on 200 mg I3C and 4 of 9 on 400 mg had complete regression of their lesions after 12 weeks, compared with none of 10 on placebo. The sample was tiny and the finding has not been replicated at scale. A 1998 open-label study of I3C in recurrent respiratory papillomatosis (warty growths in the airway caused by HPV) reported that about a third of patients stopped growing papillomas and another third had slowed growth; later follow-up supported continued use in responders. Trials in women at high risk of breast cancer confirmed I3C raises the 2:16 estrogen ratio within weeks at 300–400 mg, but no trial has tested cancer incidence as an outcome. A phase I study found dose-limiting side effects (dizziness, tremor, nausea) emerging around 800 mg daily. Research since the mid-2000s has largely shifted to DIM.
Active Compounds
Indole-3-carbinol, which converts in stomach acid to DIM, indolo[3,2-b]carbazole (ICZ), and other condensation products
Forms & Bioavailability
I3C itself is not detectable in blood after oral dosing because it converts almost entirely to DIM and other products in the stomach within minutes. Conversion efficiency depends on stomach acid, so people on proton-pump inhibitors or with low stomach acid may produce less DIM from I3C. The resulting DIM peaks in blood at about 2–3 hours. Because the metabolite mixture is variable, blood levels of active compounds after I3C are less predictable than after taking absorbable DIM directly. Taking I3C with food reduces nausea and modestly improves absorption of the lipophilic metabolites.
Dosage Guidance
| Use Case | Dosage |
|---|---|
| Estrogen metabolism support | 200–300 mg daily |
| Cervical dysplasia adjunct (studied) | 200–400 mg daily for 12 weeks |
| Recurrent respiratory papillomatosis (studied) | 200–400 mg daily, long term |
| General cruciferous support | 100–200 mg daily |
Always consult a healthcare provider for personalized dosing.
Natural Food Sources
- Brussels sprouts (among the richest in glucobrassicin)
- Broccoli, cabbage, cauliflower, kale
- Collard greens, mustard greens, bok choy
- Turnip, rutabaga, watercress, radish
- Typical serving of cooked crucifers yields a few milligrams of I3C
Potential Side Effects
Nausea, gas, skin rash, elevated liver enzymes at high doses, tremor and dizziness reported at 800 mg/day; ICZ metabolite is a potent aryl hydrocarbon receptor agonist with unknown long-term effects
Who Should Avoid It
- Hormonal contraceptives or hormone replacement therapy (may reduce effectiveness)
- Tamoxifen or other hormone-targeted cancer therapy without oncologist approval
- Liver disease or elevated liver enzymes (rare reports of liver enzyme elevation)
- Doses above 400 mg/day (neurological side effects reported at 800 mg)
- Proton-pump inhibitor use reduces conversion — DIM may be more reliable
Pregnancy & Lactation
I3C supplements are contraindicated in pregnancy and breastfeeding. Animal studies have shown both protective and tumor-promoting effects depending on timing, and the compound alters estrogen metabolism. Cruciferous vegetables as food are safe and recommended.
Known Drug Interactions
Strongly induces CYP1A1/1A2 — reduces levels of caffeine, clozapine, theophylline, and may reduce effectiveness of hormonal contraceptives and HRT; interacts with tamoxifen; may alter warfarin and acetaminophen metabolism
Evidence Classification
Based on in vitro studies, animal models, pilot trials, or traditional use documentation. Clinical evidence is limited.
Frequently Asked Questions
Should I take I3C or DIM?
DIM is the active metabolite and is more predictable, since I3C produces a variable mix of compounds including some with unclear safety. Most practitioners now favor DIM. The main reason to choose I3C is that the only randomized trial in cervical dysplasia and the respiratory papillomatosis studies used I3C specifically. Both raise the 2:16 estrogen ratio similarly.
Can I3C treat HPV or cervical dysplasia?
A very small 2000 trial found more regression of moderate-to-severe cervical lesions with I3C than placebo, but it included only 30 women and a later larger DIM trial found no benefit. I3C must never replace colposcopy, follow-up Pap tests, or treatment of high-grade lesions. If your gynecologist agrees, it can be a low-risk adjunct while monitoring low-grade changes.
Is I3C safe long-term?
Doses up to 400 mg daily for months appear well tolerated in trials, with side effects like nausea or rash in a minority. The concern is theoretical: one metabolite (ICZ) activates the same receptor as dioxin, and animal studies show I3C can promote tumors if given after a carcinogen exposure. There is no evidence of harm in humans at supplement doses, but very long-term use at high doses lacks data.
Will eating more cruciferous vegetables give the same effect?
Diet provides a few milligrams of I3C per serving, versus 200–400 mg in supplements, so dietary intake produces a smaller shift in estrogen metabolism. However, cruciferous vegetables also provide sulforaphane, fiber, and dozens of other compounds, and population studies consistently link high intake with lower cancer risk. For general health, eating crucifers regularly is the better-supported strategy; supplements are for targeted use.
References
- Placebo-controlled trial of indole-3-carbinol in the treatment of CIN. Bell MC, Crowley-Nowick P, Bradlow HL, et al.. Gynecol Oncol (2000)
- Indole-3-carbinol for recurrent respiratory papillomatosis: long-term results. Rosen CA, Bryson PC. J Voice (2004)
- Phase I trial of indole-3-carbinol in women at high risk for breast cancer. Reed GA, Peterson KS, Smith HJ, et al.. Cancer Epidemiol Biomarkers Prev (2005)
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This entry is for educational purposes only. It is not medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement regimen, especially if you take medications or have health conditions.