Anti-perspirant deodorants contain 2 compounds that have been implicated in breast cancer: Aluminum, which may interfere with the DNA, and Paraben, which is found in many other products like shampoos, creams, facial washing solutions etc and has been found to mimic estrogen, the natural hormone found in your body. The issue is that a direct cause-effect has not yet been established. What you can do is use aluminum- and paraben-free deodorants. A safe practice is to wash your arm pits everyday so as not to leave the product on your skin for more than 12 hours. And please remember that many chemicals contribute to cancer, not only the ones found in anti-perspirants, such as chemical compounds in processed foods, and industrial chemicals, to name a few.