The role of folic acid in cancer prevention is uncertain. Large observational studies have suggested a decrease in risk of colorectal and other cancers with dietary folate but others randomized trials have raised the possibility of harm.

In the largest meta–analysis of individual patient data from randomized trials of folic acid for the prevention of cardiovascular disease and colorectal adenoma 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.

Vollset SE, Clarke R, Lewington S, et al. Effects of folic acid supplementation on overall and site–specific cancer incidence during the randomised trials: meta–analyses of data on 50?000 individuals. Lancet 2013.