There isn't really any firm, conclusive evidence showing that deodorants or antiperspirants cause cancer. You can ask the National Cancer Institute . You can ask the American Cancer Society. Or, you can even ask associate professor at NYU's College of Nursing, Mei Fu, PhD, who specializes in breast cancer and its symptoms: "If we're really looking at scientific evidence," she says, "it's still inconclusive."
First off, deodorants and antiperspirants are not the same thing. When we sweat, that sweet, salty nectar is released by our sweat glands. Although sweat itself doesn’t actually smell, the bacteria that feed on our sweaty pits absolutely do. (Because sweat wasn’t gross enough on its own, apparently.) While deodorants just mask that smell, antiperspirants use an aluminum-based compound to plug up our pores and keep the sweat — and stink — at bay. When these aluminum ions make their way into the skin, they bring water with them. More water will make the cells swell and close off the pore, effectively blocking the sweat. Much of today's antiperspirant freak-outs have their roots 40 years in the past, back when studies linked aluminum to the development of Alzheimer's disease (this has since been essentially debunked).
Deodorants and antiperspirants also contain parabens, which are causing a bit of a panic in their own right. Parabens are compounds formed from organic acid and alcohol that act as preservatives in deodorant and antiperspirant. But, they’re also found in a multitude of other beauty products and, in one study, were found in the pee of 99% of participants. So, whether or not they're in your underarm-stink-fighter of choice, parabens will probably find their way to you.
However, there is plenty of other research suggesting this isn't that big of a deal. In 2002, researchers conducted a study — of 813 breast cancer patients and 793 age-matched control participants — that was published in the Journal of the National Cancer Institute. All participants were interviewed specifically about their antiperspirant use and underarm shaving. The investigators found that the majority of participants had used deodorant or antiperspirant regularly at some point, and there was no relationship whatsoever between the use of these products and a risk for breast cancer. None. Although it would be ideal to also look at a larger group of people who had never used deodorant in their lives, this is about as close as we can get; these studies took place in countries where deodorant-wearing is the norm.
Another major study, this one from the European Journal of Cancer Prevention, used retrospective survey information from 437 women with breast cancer. Results suggest that using deodorant and antiperspirants is correlated with a younger age of cancer diagnosis. But, it's important to point out here that asking people about their past is notoriously unreliable, and there was no control group in this study — meaning these results don't show conclusively that breast cancer is caused by deodorants. And, an earlier onset of puberty is a risk factor for breast cancer, and could also contribute to earlier or more frequent deodorant use.
Then there's this study, which suggests there's no link between antiperspirant use and breast cancer. And, a 2013 review of all the research available found only two studies that included a control group and provided an actual risk estimate — and neither one found an increased incidence of breast cancer in people who used antiperspirants or deodorants regularly.
So, what are we supposed to do when the science is so contradictory and incomplete? Well, Dr. Fu tells us we shouldn't sweat it. Breast cancer is multifactorial, meaning there's no one thing that will determine whether or not you get it. And, since there are plenty of lifestyle changes we know actually work to decrease your breast cancer risk (e.g., keeping a healthy diet, exercising regularly, not drinking alcohol), we should probably just focus on those. But, if you still want to try one of those super-crunchy, paraben- and aluminum-free deodorants, that's up to you. Just keep your arms down on the subway, please.