By Sharon Begley

NEW YORK (Reuters) - Although numerous studies have shown that regular use of aspirin or related drugs can reduce the risk of colorectal cancer by about 30 percent, scientists have found an important exception: The medicines can actually increase the risk in people with certain genetic variants, new research shows.

The result, published on Tuesday, is yet another step on the road to "precision medicine," which aims to match treatments to patients' genetic make-up. If confirmed, it could alter recommendations for preventing colorectal cancer, which is projected to kill 49,700 people in the United States this year.

In an editorial accompanying a paper in the Journal of the American Medical Association, Dr. Richard Wender of the American Cancer Society and Thomas Jefferson University called the discovery "scientifically noteworthy."

"I anticipate the time when genome sequencing to determine a lifelong (colorectal-cancer) prevention and screening strategy is a reality, although it's some time off," he said in an interview.

Physicians sometimes prescribe aspirin or other non-steroidal anti-inflammatory drugs for patients with a history of colon polyps. For most people, however, that is not recommended, because routine use of NSAIDs can cause gastrointestinal bleeding.

The new research, funded largely by the U.S. National Institutes of Health, aggregated the results of 10 observational studies involving 17,187 people in the United States, Canada, Australia and Germany. Regular use of aspirin or NSAIDs such as ibuprofen was associated with about 17 fewer cases of colorectal cancer per 100,000 people.

But the consequences were very different for people with some DNA variants: 34.7 additional colorectal cancer cases per 100,000 people in those with variants of one gene, and 21.1 additional cases per 100,000 in people with variants of another.

The variants are found in 4 percent to 9 percent of people of European ancestry.

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Since NSAIDs can have serious side effects - gastrointestinal bleeding can be fatal - "it's a high priority to see if we can use genetic information to target preventive interventions for individual patients," said Dr. Andrew Chan of Massachusetts General Hospital, senior researcher on the 53-author paper.

It is too soon to recommend genetic screening to guide decisions on NSAIDs for cancer prevention, at least until the results are confirmed, Chan said, "but this is a first step in that direction and toward precision medicine."

(Reporting by Sharon Begley; Editing by Lisa Von Ahn)