A study by Chicago's Rush University Medical Center says recommended dosages of folic acid may lead to faster mental deterioration in older people.
The study said older people who took folic acid, a common ingredient in multivitamins, at or above the recommended daily allowance of 400 micrograms, had a faster rate of mental decline than others their age, the New York Times reported.
The effect was also evident in people whose diets were high in folic acid.
The researchers examined more than 3,500 people 65 and older, following them an average of six years.
Martha Clare Morris, who led the latest study, did not recommend any change in diet, saying more research was needed.
"There is a possibility that high intake of folic acid through multivitamins and fortified food may be harmful to older people," she said. "It's certainly an area for further investigation."
Since 1998, the Food and Drug Administration has required all grain products be fortified with folic acid to prevent a birth abnormality called neural tube defect, the Times said.