Researchers believe they have found how aspirin helps to fight cancer.
Aspirin is sometimes called a wonder drug
UK scientists have found in laboratory tests that the painkiller can reduce the formation of the blood vessels which help to fuel tumour growth.
This could restrict the tumour to the size of a pea, the Newcastle-based team said - a finding which may eventually help to yield new treatments.
The study is published in the Journal of the Federation of American Societies for Experimental Biology.
Aspirin, often dubbed a "wonder drug", is used to treat pain, has anti-inflammatory properties and is recommended to reduce heart attack risk.
Previous studies have revealed that if it is taken over a long period of time aspirin may also be able to reduce the risk of several kinds of cancer, including breast and bowel cancer.
To investigate the possible mechanisms behind this preventative effect, scientists looked at the action of the drug on tumour growth.
They applied different concentrations of aspirin to cells that go on to form blood vessels, called endothelial cells.
They discovered that at low doses the drug had a "striking" effect on the cells' ability to form blood vessels.
Dr Helen Arthur, lead author of the paper from the Institute of Human Genetics at the University of Newcastle, said: "Blood vessels feed cancers with oxygen and nutrients, helping them grow.
"Cancer also uses new blood vessels to seed itself around the body, leading to the development of secondary cancers.
"Aspirin seems to work against tumour formation in several ways, one of which is to restrict the blood supply. Without the oxygen and nutrients supplied by blood, a solid tumour cannot grow to more than the size of a pea."
More work needed
She said this could help to explain how the drug is contributing towards cancer prevention, but it could also lead to new treatments.
"One of the problems of taking aspirin over a long period of time is that there is the risk of gastric bleeding. We want to look at the molecular targets of aspirin, the things that affect new blood vessel formation, and these may represent the opportunity for developing safer drugs."
Dr Kat Arney, Cancer Research UK's senior scientific information officer, cautioned that the researchers had only studied cells grown in the laboratory.
She said: "There is a long way to go before we know if aspirin or similar drugs can be developed into new cancer treatments, and this is just one small step along the path.
"We certainly wouldn't recommend that cancer patients take aspirin without medical advice, as large doses can be harmful."