Researchers from the Dana-Farber Cancer Institute found that for patients whose tumours harboured a mutation in the gene PIK3CA, aspirin use produced a sharp jump in survival.
Five years after diagnosis, 97 per cent of those taking aspirin were still alive, compared to 74 per cent of those not using the painkiller.
However, aspirin therapy has no effect on patients who lack the mutation, found the study involving more than 900 patients with colorectal cancer.
"Our results suggest that aspirin can be particularly effective in prolonging survival among patients whose colorectal cancer tests positive for a mutation in PIK3CA," said the study's senior author, Shuji Ogino of Dana-Farber, Brigham and Women's Hospital, and the Harvard School of Public Health.
"For the first time, we have a genetic marker that can help doctors determine which colorectal cancers are likely to respond to a particular therapy," Ogino said in a statement.
While aspirin is often prescribed for colorectal cancer patients, doctors haven't been able to predict which patients will actually benefit from the treatment.
The new finding suggests that the survival benefit is limited to the 20 per cent whose tumours have the PIK3CA mutation.
For the remaining patients, aspirin may still be used, but it is likely to be much less effective and can sometimes lead to gastrointestinal ulcers and stomach bleeding, researchers said.
The study was prompted by previous research that suggested that aspirin blocks an enzyme called PTGS2 (cyclooxygenase-2), causing a slowdown in the signaling activity of another enzyme, PI3K.
This led researchers to hypothesise that aspirin could be especially effective against colorectal cancers in which the PIK3CA gene
