Keytruda, if given at the right time and with chemotherapy, can stop an aggressive type of the disease returning.
It works by helping the immune system kill cancer cells and is already used to fight other cancers.
In a trial for The New England Journal of Medicine, Keytruda was given to women whose disease had not spread beyond the breast and lymph nodes.
Researchers found the risk of the cancer coming back was 37 per cent lower compared with those who had chemotherapy alone.
Every year around 11,500 women die from breast cancer in the UK.