Although the drugs can achieve early wins against the cancer, they also trigger a process that allows it to return and metastasize (spread) into other organs.
Researchers from the Albert Einstein College of Medicine in the US fear that other cancers would respond in a similar way to chemotherapy, and they are planning further trials to find out.
Breast cancer patients being treated with chemotherapy need to be monitored very carefully for any sign of spread, says lead researcher George Karagiannis. If it's confirmed, chemotherapy should be stopped immediately.
His team of researchers has discovered that the drugs trigger a repair mechanism that enables cancer cells to return stronger. Chemotherapy also helps create gateways, known as tumour microenvironment of metastasis (TMEM), that allow breast cancer cells to spread into other organs, which is often lethal.
Although chemotherapy can initially reduce tumour size, it also increases TMEM activity, making the cancer's spread much more likely, the researchers found when they looked at cell samples from 20 breast cancer patients. They found a similar pattern in mice.