A breakdown product of glucobrassicin from cruciferous vegetables, sold as a purified supplement. In stomach acid it condenses into a mixture of larger indoles, of which diindolylmethane is the best known.
I3C is chemically unstable in the form people swallow it — the acid of the stomach converts it into a soup of condensation products with different activities, which is why some manufacturers moved to selling DIM directly. Its research history is interesting and unresolved: it shifts oestrogen metabolism in human studies, and it has been trialled in recurrent respiratory papillomatosis and cervical intraepithelial neoplasia with some encouraging small results decades ago that were never confirmed in larger work. The animal literature is genuinely two-sided: depending on whether I3C is given before or after a carcinogen, rodent studies have shown both protective and tumour-promoting effects. That is a strong argument against casual long-term use.
Sold as
Capsule, Tablet
Standardised to
Milligrams of indole-3-carbinol — a product without this marker may not resemble what was studied.
Also known as
I3C, Indole-3-methanol
Regulation: Sold as a dietary supplement in the US with no pre-market efficacy review. Studied under investigational protocols for cervical and respiratory lesions without reaching approval.
What the research used
The amounts, forms and durations published trials of Indole-3-carbinol actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Cervical intraepithelial neoplasia
200–400 mg
Not enough
Capsule, daily · 12 weeks
From a small placebo-controlled trial in the 1990s with around thirty participants. It has not been replicated at scale and should not be treated as established.
Studied for Oestrogen metabolism studies
300–400 mg
Limited
Capsule, daily · 4–12 weeks
Doses that measurably shift urinary oestrogen metabolite ratios. The shift is the endpoint; no clinical outcome was measured.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
None of the timings below were directly compared in a trial. They reflect how the research happened to dose, or what the pharmacology implies — which is not the same as a timing being better.
🍽️With food
Not enough
Usually taken with food, both for tolerability and because acid conditions drive the conversion to its active condensation products. Not tested as a timing question.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
The evidence in depth
Best supported: Not enough
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Laboratory work shows effects on carcinogen-metabolising enzymes in both directions. Human data are limited to biomarker studies, and the animal literature is contradictory enough that no direction can be claimed.
What may be going on: I3C and its condensation products modulate aryl hydrocarbon receptor signalling and phase I and phase II enzyme expression.
Not enough evidence. Not enough human research to say anything useful yet.
I3C measurably shifts oestrogen metabolite ratios, which is why it is sold for hormone-related symptoms, but no controlled trial has connected that shift to any symptom outcome. Small early trials in cervical lesions and recurrent respiratory papillomatosis reported some regression; they were tiny, decades old, and never followed by confirmatory work.
Not enough evidence. Not enough human research to say anything useful yet.
Clinical trial · Journal of voice : official journal of the Voice Foundation · 2004 · PMID 15193659
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Worth knowing
Animal studies show effects in both directions
Rodent work has found I3C protective when given before a carcinogen and tumour-promoting when given after it, in liver, colon and thyroid models. This two-sided behaviour is unusual and is a serious argument against taking it indefinitely on the assumption it is protective.
Interaction
Induces CYP1A2 and alters drug levels
Interacts withCYP1A2 substrates including theophylline, clozapine, olanzapine and duloxetine; hormonal contraceptives; tamoxifen
I3C is a potent inducer of CYP1A2 in humans, which can substantially lower blood levels of medicines cleared by that enzyme. It also affects oestrogen metabolism, which matters for hormonal medication.
Avoid
Not for pregnancy, breastfeeding or childhood
An unstable compound that alters hormone metabolism and enzyme induction has no place in pregnancy or in children outside a supervised protocol.
Too much
Neurological effects at high doses
Higher doses in trials produced nausea, skin rash and, in a few participants, tremor and balance disturbance that resolved on stopping. Gastrointestinal upset is common.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.