London - Drugs taken by millions of women with breast cancer may not kill all malignant cells, a study suggests.
Instead hormone pills such as tamoxifen may send a small number of breast cancer cells into "sleep" mode, meaning they can come back to life up to 20 years later.
The discovery may explain why almost a third of women with the most common type of breast cancer see the disease return within a decade. It was made in a study of 50 000 breast cancer cells which found that around one in 1 000 are not destroyed but become dormant.
Experts at Imperial College London also suggest the sleeping cells are better at travelling in the body if they revive so any returning cancer is more likely to spread.
Dr Luca Magnani, who led the study published in the journal Nature Communications, said the findings could lead to new drugs which could keep dormant cells permanently asleep or kill them.
Almost three-quarters of patients have "oestrogen receptor-positive" cancer and take the hormone pills for around five years after surgery or chemotherapy.