Cancer experts are to carry out a major study to see if commonly used blood pressure drugs cut the risk of breast cancer spreading.
Data from 800 patients has already shown those previously given beta blockers had half the chance of their cancer spreading as women who had not.
So-called secondary cancers have a high death rate.
The Cancer Research UK backed study will look at about 30,000 patients, and will report next year.
If that too shows benefits from the medication, further research in which breast cancer patients would be treated with beta blockers, would follow.
Doctors cannot move straight to this kind of study because they need to have more evidence there is a beneficial effect of taking beta blockers first.
They say there is no evidence so far that anything else explains the benefits seen - but the large study will look into that even further.
Breast cancer spreading to other parts of the body is the biggest cause of death from the disease.
It is thought that about 30% of breast cancers spread, yet these account for up to 90% of all deaths from the disease.
The early work on beta blockers found that the women who had taken them had a 71% reduced risk of a cancer-related death.
Another study has also identified the biological process whereby beta blockers stop cells moving - and therefore stop cancer from spreading.
They do this by stopping the action of a molecule on the cell surface called the noradrenergic receptor. If this is blocked, cells cannot move to other parts of the body.
Dr Des Powe, from Nottingham University Hospitals NHS Trust, is working on the research in collaboration with scientists from Belfast and Germany.
He said: "Cancer can be thought of as having two distinct phases - before and after the disease has spread.
"Many women will be successfully treated for their initial breast tumour but in some, the original tumour leaves a legacy - a daughter of the primary cancer.
"It is absolutely crucial to conquer cancer spread if we are to really improve breast cancer survival as this problem causes nearly all deaths from the disease."
Dr Julie Sharp, senior science information manager at Cancer Research UK, said: "As beta blockers are already a known drug this could be a very interesting development, which has the potential to save a large number of lives."