Here is a very informative website on I3C/DIM. It says I3C has a lot of drug interaction potential as it affects CYP3A4 and others:
http://www.dimfaq.com/site/cruchoice.htm
It says this is not true of DIM.
Here are some interesting web sites.
http://www.mskcc.org/cancer-care/herb/diindolylmethane
DIM kills some cancers and prevents others from spreading. The mechanism is THROUGH affecting P450 enzymes to CLEAR ESTRADIOL FASTER.
So therefore these would be highly interactive with drug therapy. (which latter the article doesnot mention).
http://home.earthlink.net/~ckaniklidis/interactions.htm
Says that eaing broccoli 2x/day causes 40% less chance of various hormonal cancers. It works by modifying gene expression for P450 enzymes
So that estradiol is cleared fast and thus less chance of cancer. However this results in drug interactions. It says a study showed tamoxifen is Not affected by DIM but *IS* affected by I3C (rises to 3 times dose which may be toxic). the induction of CYP enzymes by I3C, especially those of the 1A subfamily, could be a cause for concern, as these play a role in activation of polycyclic aromatic hydrocarbons (PAH) and aromatic amines with known toxicities, a concern that appears not to be shared with DIM. The reason for this may be that in the acidic conditions of the stomach after oral exposure, I3C becomes a complex mixture more than 20 different I3C-derived compounds, including DIM, all having various pharmacological/toxicological effects, while DIM is relatively more stable in acid and does not robustly undergo further condensation reactions, suggesting that the more stable DIM component may be the safer compound to deploy in the human context.
Lots more in here on studies of I3C and DIM vs tamoxifen some indicating tamoxifen levels can be pretty high with no adverse effects,,,etc. Read the details.