Preventative treatment designed to stop the recurrence of breast cancer can actually cause the cancer cells to mutate and 'hibernate', only to grow again years later, according to new findings.

Researchers who set out to explain why breast cancer can return years after initial treatment have found that hormone therapies, used to prevent breast cancer from recurring, can trigger changes in some cells. These changes cause the cells to lie dormant instead of dying off, and the cells then "wake up" years later, causing a relapse that is harder to treat.

But the study found there may be a way to target these "sleeping" breast cancer cells before they "wake up.