Several large observational studies have shown a decrease in risk of colorectal and other cancers with dietary folate, while other randomized trials of folic acid supplementation have raised the possibility of increased cancer risk from folic acid supplementation.

In the largest meta-analysis of individual patient data from randomized trials of folic acid for the prevention of cardiovascular disease (10 trials, n = 49,969) and colorectal adenoma (3 trials, n = 2652), during an average of 5.2 years of treatment, there was no significant difference in overall cancer incidence for patients assigned to folic acid or placebo.

There was also no significant effect on the incidence of specific cancers, including cancers of the large intestine, prostate, lung, or breast. (Lancet 2013;381:1029)