DIM (3,3'-Diindolylmethane)
The stomach-acid condensation product of indole-3-carbinol from broccoli and cabbage, and the one supplement with genuine randomised evidence that it shifts oestrogen down the 2-hydroxy pathway rather than the 16-alpha one. What it has never shown is that the shift changes anything a woman can feel or measure clinically — and in tamoxifen users it measurably lowered endoxifen, which is a real interaction rather than a theoretical one.
Reviewed & fact-checked by
BiohackingHub Research TeamEditorial Research Team · Last updated: August 25, 2026
Medical Disclaimer: The information on this page is for educational and research purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment.
What DIM Actually Is
Chew broccoli and you release indole-3-carbinol. Swallow it, and stomach acid condenses it into a family of products dominated by 3,3'-diindolylmethane. DIM is therefore not an exotic isolate — it is what your body already makes from cruciferous vegetables, sold at a dose no realistic amount of cabbage delivers.
That framing matters both ways. It argues for a wide safety margin, and it argues against treating DIM as a hormone-grade intervention.
The Biomarker DIM Moves
Oestrone leaves the body down two competing hydroxylation routes:
- 2-hydroxyestrone (2-OHE1) — weakly oestrogenic, generally considered the benign exit
- 16-alpha-hydroxyestrone (16α-OHE1) — retains proliferative activity at the oestrogen receptor
The 2:16 ratio became a popular surrogate marker on the reasoning that pushing traffic down the 2-OH route lowers cumulative oestrogenic load. DIM does this, and the evidence for that specific claim is real.
The pilot (2004). Nineteen postmenopausal women with a history of early-stage breast cancer took absorption-enhanced DIM at 108 mg/day for 30 days. Urinary 2-OHE1 rose significantly (P = 0.020). The 2:16 ratio rose 47%, from 1.46 to 2.14 — but at P = 0.059, that headline number did not reach significance. Nineteen women is a pilot, and it should be read as one.
The proper trial (2017). A randomised, placebo-controlled, 12-month study gave 150 mg twice daily of absorption-enhanced DIM to women taking tamoxifen; 98 of 130 completed. The 2:16 ratio rose +3.2 on DIM versus −0.7 on placebo (P < 0.001). SHBG rose by about 25 nmol/L against 1.1 on placebo. Adverse events were minimal and did not differ between arms.
So: DIM changes oestrogen metabolism in humans, at a real dose, over a real duration, with a real control group. That is more than most supplements in this category can claim.
What DIM Has Never Shown
The same 2017 trial is where the honest reading turns.
Breast density did not change — not on mammography, not on MRI. The metabolite ratio moved and the tissue endpoint did not follow. And the 2:16 ratio's own status as a risk marker is contested; it is a biomarker of plausible interest, not a validated surrogate for clinical outcome.
Then the finding that belongs on every DIM label: plasma tamoxifen metabolites — endoxifen, 4-OH tamoxifen and N-desmethyl-tamoxifen — were significantly reduced in the DIM arm (P < 0.001). Endoxifen is the metabolite that does most of tamoxifen's work. The trial authors explicitly called for further research into whether this attenuates tamoxifen's clinical benefit.
A supplement taken specifically by women worried about oestrogen-driven disease measurably lowered the active metabolite of the drug that treats it. That is not a theoretical interaction, and it is the single most important thing to know about DIM.
Dosing and the 200 mg Ceiling
A single-ascending-dose study in healthy volunteers established the practical envelope for absorption-enhanced (BR-)DIM:
| Single dose | Mean Cmax | Notes |
|---|---|---|
| 50 mg | Undetectable in all but one subject | Below the useful threshold |
| 100 mg | 32 ng/mL | Detectable, modest |
| 200 mg | 104 ng/mL | No adverse effects reported up to here |
| 300 mg | 108 ng/mL | Nausea, headache, one vomiting episode — and no gain in peak concentration |
Exposure plateaus between 200 and 300 mg while tolerability degrades. 150 mg/day of an absorption-enhanced form, taken with fat, split if above 150 mg is the sensible target. Plain DIM without an absorption-enhanced delivery system is poorly absorbed and its labelled milligrams are not comparable.
Expect darkened urine. It is a DIM artefact and it alarms people who were not warned.
Who This Is Reasonable For
DIM is a mechanism-level intervention with a biomarker-level evidence base. That makes it defensible for a woman in the menopause transition who wants to support oestrogen clearance alongside a foundation that has actual outcome data — and indefensible as a treatment for anything.
It is not appropriate on tamoxifen, in pregnancy, or as a substitute for evaluation of abnormal bleeding, breast changes, or any symptom that deserves a diagnosis rather than a supplement.
Related Research
Stacking Interactions
How DIM (3,3'-Diindolylmethane) interacts with other compounds
The standard pairing, and mechanistically coherent: DIM works phase I (which metabolite gets made), calcium-D-glucarate works the phase III exit (keeping conjugated oestrogen from being deconjugated and reabsorbed). Coherent does not mean proven — the combination has no clinical outcome trial
Protocols using DIM (3,3'-Diindolylmethane)
Evidence-graded stacks that include this compound
Safety Profile — Tier B
Generally safe — moderate evidence
Contraindications
- ●Concurrent tamoxifen — a 12-month RCT found significantly reduced plasma endoxifen and other tamoxifen metabolites
- ●Pregnancy and breastfeeding — no adequate data
Side Effects
- ●Harmless darkening of urine (a known DIM artefact, not haematuria)
- ●Nausea, headache and occasional vomiting at single doses of 300 mg
- ●Headache and gastrointestinal upset at higher chronic doses