Are there reasons to suspect that it could be dangerous to take silibinin/sylimarin in combination with standard chemotherapy?


http://informahealthcare.com/doi...
Silibinin has clearly demonstrated inhibition of multiple cancer cell signaling pathways, including growth inhibition, inhibition of angiogenesis, chemosensitization, and inhibition of invasion and metastasis. Cumulative evidence implicates that silibinin is a potential agent for cancer chemoprevention and chemotherapy.

Marc Pimentel, Resident MD

The is answer is unclear. Resources from the National Center for Complementary and Alternative Medicine and Memorial Sloan-Kettering Cancer Center note that milk thistle is being studied, but data about its clinical effects are inconclusive. 
Always talk to your healthcare provider about any herbal supplements that you are taking, as they may have serious drug interactions with your current medication regimen.
http://nccam.nih.gov/health/milk...
http://www.mskcc.org/mskcc/html/...