Researchers in the US studied the impact of drugs on patients with breast cancer and found medication increases the chance of cancer cells migrating to other parts of the body, where they are almost always lethal.
Around 55,000 women are diagnosed with breast cancer in Britain every year and 11,000 will die from their illness.
Many are given chemotherapy before surgery, but the new research suggests that, although it shrinks tumours in the short term, it could trigger the spread of cancer cells around the body.
It is thought the toxic medication switches on a repair mechanism in the body which ultimately allows tumours to grow back stronger. It also increases the number of 'doorways' on blood vessels which allow cancer to spread throughout the body.