Is there evidence that oral contraceptive pills are less effective in pre-menopausal women on tamoxifen chemoprevention?  

While I am not aware of such study, going back to the history of how the tamoxifen, compound ICI 46,474 was developed as an oral contraceptive, as well as numerous evidence of estrogen-mimicking properties we observed in the laboratory, one can safely assume that tamoxifen will certainly interfere w...