New studies show that high doses of folic acid may raise the risk of prostate cancer. A 10 year study performed on 643 men taking 1000 mcg of folic acid to show if the B vitamin could prevent the occurrence of precancerous colon polyps, instead revealed a 10% increase in the incidence of prostate cancer. Though the study can be considered inconclusive due to the size of the group, it doesn't hurt to play it safe while additional studies continue.
What to do? If you take a multivitamin containing 400 mcg (100% of the daily value), take only one every-other day. Also, watch for fortified breakfast cereals (should contain no more than 25% of the daily value). For more information on folate fortified foods, refere to the article on Breast Cancer & Folate from Oct. 2009 archives. There's no need to limit fruits and vegetables and other folate rich foods.
~National Cancer Institute, 2009~