CHICAGO — High doses of folic acid do not prevent precancerous colon polyps in people prone to them and may actually increase the risk of developing the growths, a new study finds.

It's the latest evidence that taking too many vitamins may be harmful. Last month, a study linked heavy vitamin use to fatal prostate cancer, and other research has shown beta-carotene pills can heighten smokers' risk of lung cancer.

The results surprised scientists. Previous studies showed diets low in folic acid led to a higher risk of colon cancer.

Now researchers speculate that some folic acid helps — as long as the colon is free of microscopic cancer cells. But once cancer starts, folic acid may feed its growth.

Some scientists who reviewed the new findings said folic acid fortification, now required in some U.S. foods, should not be increased and that other nations considering fortification should be cautious.

The new findings, appearing in Wednesday's Journal of the American Medical Association, are based on data from 987 adults with a history of precancerous colon polyps. Those who took folic acid developed more growths, or adenomas, several years later than the people who took dummy pills.

"You really should not take folic acid to prevent colorectal adenomas. It's ineffective for that purpose," said study co-author Bernard Cole of Dartmouth-Hitchcock Medical Center.

Folic acid is an artificial version of folate, a B vitamin found in leafy vegetables, citrus fruit and beans. It prevents birth defects and is needed for the production of red blood cells.

In the study, participants randomly were assigned to take either folic acid or a dummy pill. Researchers followed them for about six years.

Participants got screening colonoscopies a few years into the study and 44.1 percent of the folic-acid takers had precancerous polyps. That compared to 42.4 percent of the dummy-pill group.

The difference was not statistically significant, but the results of a second round of colonoscopies a few years later were more troubling. Among the folic-acid takers, 11.6 percent had advanced adenomas while 6.9 percent of the dummy-pill group did. And folic acid more than doubled the risk of having three or more precancerous polyps.

For those who got the real vitamin, the daily dose was 1 milligram, more than double the recommended daily allowance for folic acid. All participants consumed even more folic acid than the researchers had in mind because the Food and Drug Administration began requiring enriched grains to be fortified with folic acid in 1998, several years after the study began.

The FDA adopted the fortification policy to prevent birth defects, and some health advocates want even higher levels of folic acid in foods. But fortification may have unintended effects on people at risk of cancer, said Dr. Joel Mason, an expert on folate and cancer prevention at Tufts University in Boston who was not involved in the new study.