Though arsenic, which found in drinking water, has been linked to harmful effects such as arsenic poisoning, a new study out of the University of California, Berkeley, and the Pontifical Catholic University of Chile has found that it might somehow assist in the fight against breast cancer.

The results of the study may appear somewhat contradictory to the research that already exists about arsenic’s health hazards. Arsenic is a naturally occurring chemical element that is often found in various minerals, alongside sulfur and metals. When it’s found in drinking water, however, it can lead to arsenic poisoning, and even increase people’s risk of cancer.

The authors of the Berkeley study examined breast cancer mortality data from a region in Chile that is notorious for having high levels of arsenic in the drinking water and environment. Scientists normally see an increase in cancer and toxicity in such regions, but this study shows opposite results. Breast cancer deaths were actually cut in half in this Chilean region during a period of time that coincided with high arsenic exposure. Women under the age of 60 had the most pronounced results; breast cancer mortality was cut by 70 percent.

“What we found was astonishing,” said Dr. Allan Smith, lead author of the study and a professor of epidemiology at Berkeley, in a press release. “We’ve been studying the long-term effects of arsenic in this population for many years, focusing on increased disease and mortality attributed to the historical exposure to arsenic in this population.” Once an arsenic removal plant was placed in the city of Antofagasta, Chile, the researchers found that breast cancer deaths began to rise again.

While exposure to arsenic has been linked to the growth of malignant cells for various other types of cancer such as lung cancer, breast cancer cells were shown to be destroyed by arsenic. Stanford Cancer Institute researchers assisted in the study by growing breast cancer cells in the lab, and finding that arsenic in fact killed them. Regular breast cells, meanwhile, appeared to be more resistant to arsenic than other types of cells.

A Double-Edged Sword

Interestingly enough, though arsenic exposure can be toxic, the element has been used medically for over 2,400 years to treat various conditions. The element has been referred to as a medicinal “double-edged sword,” due to its ability to work as a therapeutic agent as well as a poison. Arsenic trioxide has often been used to treat relapsed promyelocytic leukemia, for example. But it has also been found to cause certain types of cancers, though more research is needed to understand how.

Despite all this, arsenic appears to be different in the case of breast cancer. The element may be used to treat breast cancer one day, since “arsenic has been shown to cause induction of functional re-expression of the estrogen receptor in estrogen negative breast cancer cells, which could make them less aggressive,” the authors wrote. But Smith and his colleagues are hesitant about believing that arsenic can be used to treat breast cancer, and more research will need to be done to identify how the "double-edged sword" can be used against breast cancer without causing other harmful effects. “We do not know if the treatment will work, but carefully designed clinical trials should take place as soon as possible based on this new evidence.”

Source: Smith A, Marshall G, Yuan Y, Steinmaus C, Liaw J, Smith M. Rapid Reduction in Breast Cancer Mortality with Inorganic Arsenic in Drinking Water. EBioMedicine, 2014.