For most people, a regular dose of aspirin, Advil, Aleve or certain other over-the-counter painkillers can reduce the risk of colorectal cancer by about one-third. But for some people, these same pills make colorectal cancer more likely.

Now researchers have figured out a way to tell these two groups apart by looking at three specific spots in the vast human genome.

After combing through the DNA of more than 17,000 people in four countries, the researchers identified a few genetic variants that appear to influence whether drugs like aspirin increase or decrease one’s risk of colorectal cancer. Their findings were published by the Journal of the American Medical Association.

The study is an example of how “big science” can untangle the influence of genetics and the environment and show how the two interact to cause — or prevent — diseases, according to Dr. Richard C. Wender, the chief cancer control officer for the American Cancer Society in Atlanta.

“The ability to translate genetic profiling into tailored preventive care plans for individuals is still years away,” Wender wrote in an editorial that accompanies the JAMA report. But the analysis gives scientists a clearer picture of how to get there from here, he added.

Researchers identified three intriguing single nucleotide polymorphisms, or SNPs, on the human genome. When people had the common versions of these SNPs, taking the painkillers was associated with a 34 percent reduced risk of colorectal cancer. But when people had uncommon versions of these SNPs, taking the drugs offered no benefit — or else increased the risk of colorectal cancer.