From the National Cancer Institute:

In 2002, the results of a study looking for a relationship between breast cancer and underarm antiperspirants/deodorants were reported (3). This study did not show any increased risk for breast cancer in women who reported using an underarm antiperspirant or deodorant. The results also showed no increased breast cancer risk for women who reported using a blade (nonelectric) razor and an underarm antiperspirant or deodorant, or for women who reported using an underarm antiperspirant or deodorant within 1 hour of shaving with a blade razor. These conclusions were based on interviews with 813 women with breast cancer and 793 women with no history of breast cancer.

A different study examining the frequency of underarm shaving and antiperspirant/deodorant use among 437 breast cancer survivors was released in 2003 (4). This study found that the age of breast cancer diagnosis was significantly lower in women who used these products and shaved their underarms more frequently. Furthermore, women who began both of these underarm hygiene habits before 16 years of age were diagnosed with breast cancer at an earlier age than those who began these habits later. While these results suggest that underarm shaving with the use of antiperspirants/deodorants may be related to breast cancer, it does not demonstrate a conclusive link between these underarm hygiene habits and breast cancer. Additional research is needed to investigate this relationship and other factors that may be involved.

Other research has focused on certain preservatives (parabens) that are used in deodorants and antiperspirants, as well as many cosmetic, food, and pharmaceutical products. Parabens have been shown to mimic the activity of estrogen (a hormone) in the body’s cells (5). Because estrogen promotes the growth of breast cancer cells, some scientists have suggested that the use of deodorants or antiperspirants could cause the accumulation of parabens in breast tissues, which may contribute to the development of breast cancer. This hypothesis was supported by a 2004 study that found parabens within 18 of 20 samples of tissue from human breast tumors (6). However, this study did not prove that parabens cause breast tumors (5). The authors of this study did not analyze healthy breast tissue or tissues from other areas of the body, and did not demonstrate that parabens are found only in cancerous breast tissue (6). Furthermore, this research did not identify the source of the parabens, and cannot establish that the accumulation of parabens is due to the use of deodorants or antiperspirants. More research is needed to specifically assess whether the use of deodorants or antiperspirants can cause the accumulation of parabens in breast tissue, and whether these chemicals can promote the development of breast cancer.