A study by Kim et al, published in the Journal of Toxicology and Environmental Health, found that phthalates significantly increased tumor cell proliferation. In addition, the phthalates tested mimicked estrogen in the inhibition of tamoxifen-induced apoptosis (cell death) in some tumor cells.

Their data suggests that phthalates, which are estrogen mimics, inhibit the effectiveness of tamoxifen.  From the study:  “Given that the phthalates are widely used in cosmetics mainly for women, our findings that revealed the promoting effect of BBP, DBP, and DEHP [[types of phthalates]] on chemotherapeutic drug resistance to tamoxifen in breast cancer may be of biological relevance.”