A daily dose of aspirin significantly improves the test results of men with prostate cancer, research has shown.
Taking the pill causes their PSA level to fall, suggesting that it is helping the body fight the disease.
PSA, or prostate specific antigen, is a blood marker used to monitor the progress of prostate cancer.
However scientists are still not certain that aspirin really does protect against the disease which kills about 10,000 men in the UK each year.
The possibility remains that it might be misleading by lowering PSA without affecting the disease.
If this true, it could also mean that aspirin is allowing some cancers to go undetected.
The research involved 1,277 men taking part in a large U.S. investigation called the Nashville Men's health study.
About 46 percent of the men were regularly taking a non-steroidal anti-inflammatory drug (NSAID) such as ibuprofen or aspirin. The vast majority, around 37 percent, took aspirin.
Aspirin use led to 9 percent lower PSA levels, the scientists reported at an international meeting of the American Association for Cancer Research in Washington DC.
The effect was strongest in men with prostate cancer, suggesting that it was having an impact on the disease.
PSA tests are widely used as the first step in the process of diagnosing prostate cancer. A high reading may indicate cancer, but can also be caused by a benign condition. A biopsy -- the removal of a small piece of tissue for examination -- is needed to confirm that cancer is present.
All the men taking part in the study had been referred to a urologist to have prostate biopsies. Dr. Jay Fowke, from Vanderbilt University in Nashville, Tennessee, who led the research, said: "To begin to understand how aspirin may lower PSA, we also looked at the association between NSAID use and prostate volume. "Aspirin users and men who didn't use aspirin had the same prostate volume, so I don't think aspirin was changing PSA by changing the prostate volume. It was doing something different, and that suggests a beneficial effect on cancer development." Several previous studies had reported an association between anti- inflammatory drugs such as aspirin and reduced prostate cancer risk, said Dr Fowke. "Our data also suggest that NSAID use has a beneficial effect on prostate cancer," he added. "These findings could be consistent with a protective effect, because aspirin reduced PSA levels more among those men who were diagnosed with prostate cancer than among men with other prostate diseases." But he warned it was also possible that using NSAIDs could affect the ability of doctors to detect prostate cancer. "This analysis raises the concern that aspirin and other NSAIDs may lower PSA levels below the level of clinical suspicion without having any effect on prostate cancer development, and if that is true, use of these agents could be hampering our ability to detect early-stage prostate cancer through PSA screening," he said.
