Aspirin, a painkiller and anti-inflammatory, is widely prescribed to people at risk of heart disease.
In patients whose blood vessels are narrowed from atherosclerosis, or 'furring' of the arteries, fatty deposits on the lining can cause a blocked artery and a heart attack.
Aspirin works by interfering with blood clotting by reducing the clumping together of platelets or clotting cells.
Studies have also suggested it may help prevent colorectal cancers in heart patients.
One theory about its anti-cancer effect is that the drug blocks cyclooxygenase or COX enzymes which are involved in the pain and inflammatory process.
COX enzymes may also be involved in the growth of new blood vessels that help cancerous tumours to grow, and that by blocking their activity, aspirin reduces the likelihood of the cancer both growing and also spreading.
Aspirin may trigger cancer cell death too.
There are nearly 50,000 new cases of prostate cancer a year and 11,000 deaths, according to Cancer Research UK.
Almost nine out of ten men diagnosed with prostate cancer survive the disease for five years or more.
The prostate study was carried out by doctors from the Italian College of General Practitioners and Primary Care and reported in the International Journal Of Cancer.