From the Guidelines

Diindolylmethane (DIM) has chemopreventive activities in all stages of breast cancer carcinogenesis.

DIM is a compound found in cruciferous vegetables and has been shown to have antiproliferative and antioxidant activity in breast cancer cells 1.
The hydrolysis of some compounds found in cruciferous vegetables may yield DIM, which has demonstrated chemopreventive activities in all stages of breast cancer carcinogenesis 1.
DIM has been suggested to be a reliable biomarker of glucobrassicin exposure and indole-3-carbinol uptake 1.
A daily intake of 150 g cooked cruciferous vegetables for 4 weeks had no effect on thyroid function, suggesting that consuming cruciferous vegetables is safe and may be beneficial for breast cancer prevention 1.
The American Institute of Cancer Research, the American Cancer Society, and the NIH have all issued guidelines in support of increasing fruit and vegetable intake, which includes cruciferous vegetables that contain DIM, to reduce the risk of breast cancer recurrence 1.

From the Research

The benefits of Diindolylmethane (DIM) can be summarized as follows:

DIM has been shown to exhibit significant metabolism after oral dosing in humans, with two monohydroxylated metabolites and one dihydroxylated metabolite present in both plasma and urine 2
DIM has been found to promote metabolic crisis and enhance the efficacy of Centchroman in breast cancer, by regulating aerobic glycolysis and intermediate metabolism 3
DIM has been shown to increase the 2-hydroxylation of estrogen urinary metabolites, which may provide potential protection against hormone-dependent cancers 4
DIM has been found to be a strong androgen antagonist in human prostate cancer cells, suppressing cell proliferation and inhibiting dihydrotestosterone stimulation of DNA synthesis 5
However, low levels of DIM have been shown to activate estrogen receptor α and induce proliferation of breast cancer cells in the absence of estradiol, which may have unexpected effects on cell growth 6

The potential therapeutic applications of DIM can be summarized as follows:

DIM may be used as an adjuvant therapy for treating metabolically dysregulated triple-negative breast cancer (TNBC) 3
DIM may be used to prevent or treat hormone-dependent cancers, such as prostate cancer and breast cancer 4, 5
However, the use of DIM as a dietary supplement or nutraceutical should be approached with caution, as it may have unexpected effects on cell growth and hormone regulation 6

The mechanisms of action of DIM can be summarized as follows:

DIM has been shown to regulate aerobic glycolysis and intermediate metabolism in TNBC cells 3
DIM has been found to inhibit androgen-induced androgen receptor translocation into the nucleus, and to compete with dihydrotestosterone for binding to the androgen receptor 5
DIM has been shown to activate estrogen receptor α and induce proliferation of breast cancer cells in the absence of estradiol, through a mechanism that involves the protein kinase A signaling pathway 6