DIM (diindolylmethane)

What it is

Cruciferous vegetables such as broccoli, cabbage and Brussels sprouts contain glucobrassicin. The plant’s own enzyme myrosinase or gut bacteria split this mainly into indole-3-carbinol, and in the acidic stomach two of these molecules at a time combine to form DIM. DIM is therefore a substance that humans have always taken in with vegetables. To reach the blood levels that capsules deliver, however, one would have to eat kilogram quantities of Brussels sprouts daily, according to a review.

Studies almost always use a form with improved absorption called BR-DIM. In one analysis, a capsule with 150 mg BR-DIM contained 45.3 mg DIM. Anyone comparing studies must therefore look closely at whether a quantity refers to the product or to the active substance.

How it is supposed to work

Estrogens are broken down in the liver along different routes. One route leads to 2-hydroxy estrogens with little hormonal activity, another to 16α-hydroxy estrogens, which retain their estrogenic activity. DIM activates the aryl hydrocarbon receptor and with it the enzymes CYP1A1 and CYP1B1. CYP1A1 converts estrogens mainly at position 2, CYP1B1 mainly at position 4. On balance, the ratio shifts in favor of the 2-hydroxy estrogens. The ratio of the two breakdown products in the urine is the marker by which almost all DIM studies are measured. In a one-year study with 23 BRCA carriers, estradiol also fell from 159 to 102 pmol/l and testosterone from 0.42 to 0.31 pmol/l on 100 mg DIM per day, though without a placebo group.

In the laboratory, DIM additionally acts as an antagonist at the androgen receptor. In prostate cancer cells it displaced dihydrotestosterone from its binding site, slowed cell growth and lowered PSA production. The authors therefore described DIM as a hormone modulator with two faces. In humans, DIM is also metabolized faster and in more ways than long assumed: several breakdown products appear in the blood, and one of them acts even more strongly at the aryl hydrocarbon receptor than DIM itself.

What users are looking for

Women take DIM for complaints they attribute to estrogen dominance, for hormonal acne or during menopause. Men in strength sports use it to lower estrogen and support testosterone. DIM is now one of the most common ingredients of acne supplements. Many users report clearer skin or calmer cycles. These are uncontrolled user reports that can be taken seriously, but that have not yet been tested in studies.

What the studies show

The tamoxifen study

130 women on tamoxifen received 150 mg BR-DIM twice daily or placebo for 12 months; 98 completed the study. DIM markedly shifted estrogen breakdown and raised SHBG; breast density on mammography and MRI remained unchanged. Unexpectedly, blood levels of endoxifen and other active tamoxifen breakdown products fell. Whether this weakens the protection provided by tamoxifen is an open question.

The cervical study

551 women with newly detected mild cervical cell changes received 150 mg DIM or placebo for 6 months. A precancerous lesion of grade CIN2 or worse was then found in 9 percent on DIM and 12 percent on placebo, a difference within the range of chance. DIM had no influence on the persistence of HPV infection. The authors consider an effect on cytology and HPV unlikely; for the effect on CIN2 and worse, uncertainty remains.

The study with young women

In Mexico, 60 premenopausal women with an unfavorable estrogen profile received 75 mg DIM or placebo for 30 days. The ratio of estrogen metabolites did not rise significantly; the primary endpoint was missed. Surprisingly, body fat percentage fell more on DIM than on placebo, a secondary finding that still needs to be confirmed.

Where the data stop

The ratio of estrogen metabolites is a laboratory value. No study has yet shown whether shifting it prevents breast cancer, relieves cycle complaints or improves acne. Where clinical endpoints were measured, the effect was absent or not significant: for cervical cell changes in two randomized studies, for breast density in the tamoxifen study. A single-arm study with 23 BRCA carriers saw somewhat less glandular tissue on MRI after one year, but had no placebo group. The good results for cell changes come from a study with vaginal suppositories and cannot be transferred to capsules.

For the purposes popular in the scene, controlled data are missing entirely. On testosterone in men there is one single case report, on acne not a single randomized study, on fat loss only the secondary finding from Mexico. Most studies are small, short or both.

The animal data are also not as clear-cut as the advertising suggests. In a rat breast cancer model, the precursor indole-3-carbinol was protective, but DIM was not. In rainbow trout, DIM even promoted liver cancer because it acted there as a strong phytoestrogen. That is not a finding in humans, but it shows that DIM can act in different directions depending on tissue and hormonal status.

Status, approval and legal

In the EU, DIM is treated as an unauthorised novel food. The European rapid alert system RASFF has recorded several notifications on food supplements containing DIM since 2021, most recently in August 2026. In Germany, DIM therefore cannot legally be marketed as a food supplement, and there is no approved medicine either. In the US, DIM has been sold as a dietary supplement for years and is at the same time being studied as an investigational substance in clinical trials. There are no reference values or maximum levels from EFSA or BfR. DIM is not named on the WADA list for 2026.

Safety

At study doses over six to twelve months, DIM was well tolerated. At higher single doses, individual participants reported nausea, headache and vomiting, and in a dose-finding study with prostate cancer patients, the sodium level dropped markedly in two of four men at the highest level. The most important points are interactions. On tamoxifen, the levels of the active breakdown products fell. In women with estradiol patches, DIM changed 6 of 10 measured estrogen metabolites, which could weaken hormone therapy. In cell experiments, DIM stimulates the enzymes CYP3A4 and P-glycoprotein, through which many medicines are broken down. Anyone taking hormones, tamoxifen or other long-term medication should use DIM only after consulting a doctor. Data are missing for pregnant and breastfeeding women and for people with hormone-dependent diseases.

Sources

• Thomson et al., Breast Cancer Res Treat 2017 – DIM during tamoxifen, 12-month RCT
• Castañon et al., Br J Cancer 2012 – DIM in mild cervical cell changes
• Del Priore et al., Gynecol Oncol 2010 – oral DIM in CIN 2 and 3
• Dalessandri et al., Nutr Cancer 2004 – DIM and estrogen metabolites after breast cancer
• Godínez-Martínez et al., Nutr Cancer 2023 – DIM in premenopausal women
• Le et al., J Biol Chem 2003 – DIM as an androgen receptor antagonist
• Heath et al., Am J Transl Res 2010 – phase I dose escalation in prostate cancer
• Newman and Smeaton, Menopause 2025 – DIM and estradiol patches
• Williams, Front Nutr 2021 – indole-3-carbinol and DIM, review
• RASFF Window – notifications on DIM as an unauthorised novel food