Aspirin apparently exerts a protective effect only in people with high levels of an enzyme called 15-PGDH, which is found in the gut's lining.
Researchers tracked aspirin use among 127,865 participants in two large national health surveys and found 270 cancer cases in which 15-PGDH levels were tracked. The study appears online in Science Translational Medicine.
They found that taking two regular-strength tablets a week had no significant effect on the risk of cancer in people with low levels of the enzyme. But in people with high levels, it cut the risk by 50 percent. The risk decreased with higher doses and longer duration of aspirin use.
The effect persisted after controlling for many health and behavioral factors, including the use of other nonsteroidal anti-inflammatory drugs like ibuprofen (Advil) and naproxen (Aleve).
Doctors don't routinely test for 15-PGDH. But levels can be determined safely and inexpensively by doing an additional biopsy during a colonoscopy, said the senior author, Dr. Andrew T. Chan, an associate professor of medicine at Harvard.
"There is a real need to identify who will benefit from aspirin use," said Dr. Chan, "and not introduce risk" — such as gastrointestinal bleeding — "to people who will not."