Like any normal person should, I applied antiperspirant/deodorant every day before I left for school. One morning, however, my mother stopped me saying, "If you wear that too much you could get breast cancer, you know." When she said this I laughed and continued to put on my deodorant, but as the day went on I was curious to see if what she said was really true.
One principal we can use to out rule this hypothesis is correlation vs. causation. Just because most all woman that test positive for breast cancer are wearing antiperspirant/deodorant, does not mean that it is the cause of the cancer. According to the American Cancer Society, all the claims of this myth are false. These claim most often heard is that, "Underarm shaving allows cancer-causing substances in antiperspirants to be absorbed through razor nicks." People believed that the cuts or holes in your skin that women receive from shaving would allow certain chemicals from the antiperspirant/deodorant to seep into the lymph nodes under your arm, creating toxins that would later cause cancer. However, the National Cancer Institute claims that there must be a different cause for breast cancer, because there is no concrete evidence linking the two together.
In 2002, results from a study were released about this topic. Scientists interviewed 813 women with breast cancer and 793 woman without, finding no correlation between antiperspirant/deodorant and cancer. Another study was conducted finding the shocking result that women who were diagnosed at an earlier age were using antiperspirant/deodorant then ones who were diagnosed at a later age. However, the American Cancer Society says, "Probably, in general, younger women are more likely than older women to shave their underarms and use antiperspirants, whether or not they develop breast cancer later." It is partially just our generation. We are more exposed to using these hygiene products than older women were when they were our age. So when we look at the big picture, there is no correlation between antiperspirant/deodorant and the cause of breast cancer. This is why the principal of correlation vs causation is the most effective way to evaluate this false claim.
National Cancer Institute
American Cancer Society