Indole-3-Carbinol (I3C)

non-nutrient/non-botanicalindole-3-methanol

What is it

Indole-3-carbinol (I3C) is a compound produced when cruciferous vegetables (broccoli, cabbage, Brussels sprouts) are chopped, chewed, or otherwise damaged, releasing it from its glucosinolate precursor glucobrassicin. In the stomach, I3C converts to diindolylmethane (DIM) and other compounds.

How it works

I3C is unstable in stomach acid and rapidly converts to a mixture of acid-condensation products, with diindolylmethane (DIM) being the most abundant. The biological effects attributed to I3C are largely mediated by DIM and related condensation products. These compounds modulate estrogen metabolism by promoting hydroxylation at the C-2 position of the estradiol molecule, producing 2-hydroxyestrone (considered a less hormonally active metabolite) and reducing the formation of 16-alpha-hydroxyestrone. This shift is the primary mechanism proposed for I3C's effects on hormone-related conditions. Research suggests I3C and its metabolites also activate the aryl hydrocarbon receptor (AhR), influencing phase I and phase II detoxification enzymes. Preclinical evidence shows effects on cell cycle, apoptosis, and inflammation. Higher doses of I3C have shown both beneficial and adverse effects in animal models, with some studies suggesting tumor-promoting effects in certain contexts, raising caution about high-dose long-term use.

Evidence for 4 uses

AI-assisted evidence assessment — talk to your doctor before relying on any single supplement.

Estrogen metabolism

Grade C

Moderate evidence

Several trials show I3C supplementation increases the ratio of 2-hydroxyestrone to 16-alpha-hydroxyestrone in urine. Clinical significance for disease outcomes is uncertain.

Recurrent respiratory papillomatosis

Grade C

Moderate evidence

Small case series and limited trials suggest I3C may reduce the recurrence of HPV-related laryngeal papillomas in some patients.

Cervical dysplasia

Grade D

Mixed evidence

Small trials have investigated I3C for cervical intraepithelial neoplasia with mixed results. Not standard of care.

Breast cancer prevention

Grade F

Limited evidence

Mechanistic plausibility from estrogen metabolism effects, but no clinical trial has demonstrated breast cancer prevention from I3C supplementation.

2 commercial forms

I3C capsules

I3C is unstable in stomach acid; absorption involves conversion to multiple products.

Common form. Stability and conversion are unpredictable, leading many practitioners to prefer DIM instead.

I3C combined with DIM

Provides both precursor and primary metabolite.

Marketed as providing comprehensive estrogen metabolism support.

Dosage

Typical supplement doses are 200-400 mg per day of I3C, sometimes higher in clinical studies. There is no established RDA. Many practitioners now prefer DIM over I3C due to its stability and more predictable conversion. I3C should be taken with food.

When and how to take it

Take I3C with a meal containing some fat to support absorption. Most users take it once or twice daily. Many practitioners prefer DIM supplements over I3C for more predictable effects. Consistent daily use over weeks is needed to evaluate hormonal effects.

Food sources

Food	Amount	%DV
Broccoli (1 cup cooked)	approx 100 mg I3C equivalent	—
Brussels sprouts (1 cup)	Higher I3C content among crucifers	—
Cabbage, raw (1 cup)	approx 75 mg I3C equivalent	—
Kale (1 cup cooked)	Significant I3C content	—
Cauliflower (1 cup)	Moderate I3C content	—

Safety

I3C is generally well tolerated at typical doses. Common side effects include nausea, gastrointestinal upset, and headache. Higher doses may cause transient rashes, dizziness, and tremor. Animal studies have shown both protective and tumor-promoting effects in different cancer models, raising concerns about high-dose long-term use.

Who should be cautious

Avoid in pregnancy and breastfeeding due to insufficient safety data and hormonal effects. People with hormone-sensitive cancers should consult their oncologist before use. Women on hormonal contraception or HRT should consult a clinician. Avoid high doses (above 400 mg per day) without medical supervision. People with liver disease should use cautiously.

Interactions

I3C induces CYP1A1, CYP1A2, and other detoxification enzymes, potentially affecting the metabolism of medications cleared by these enzymes, including caffeine, theophylline, certain antidepressants, beta-blockers, and others. I3C may affect the metabolism of hormonal medications including birth control and HRT. May also affect tamoxifen metabolism.

Frequently asked questions

Should I take I3C or DIM?⌄

DIM is the more stable downstream product and is often preferred for predictable effects. I3C converts to DIM and other compounds in the stomach, with results that can vary. Many practitioners recommend DIM instead.

How much I3C is in cruciferous vegetables?⌄

A cup of cooked broccoli or Brussels sprouts provides roughly 50-100 mg I3C equivalent. Supplements typically deliver several times this amount.

Is I3C safe for long-term use?⌄

Long-term safety data are limited, and animal studies have raised concerns about high-dose use. Short-term use at typical doses is generally well tolerated.

Can I3C affect my birth control?⌄

I3C induces enzymes that metabolize hormonal contraceptives and may reduce their effectiveness. Consult your clinician about potential interactions.

Why does I3C cause some people to feel different effects each time?⌄

I3C is unstable in stomach acid and converts to a variable mixture of products depending on individual stomach conditions, leading to inconsistent effects. DIM provides more predictable activity.

References

Wikidata: Indole-3-carbinol — Wikidata link

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Disclaimer: These statements have not been evaluated by the FDA. This page is educational, not a substitute for personalized medical advice. Evidence grades are AI-assisted assessments — talk to your doctor before starting any new supplement, especially if you're pregnant, breastfeeding, on medications, or managing a chronic condition.