"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 Jan 25, 2013
http://www.lancet.com/journals/lancet/article/PIIS0140-6736(12)62001-7/fulltext
"Interpretation

Folic acid supplementation does not substantially increase or decrease incidence of site-specific cancer during the first 5 years of treatment. Fortification of flour and other cereal products involves doses of folic acid that are, on average, an order of magnitude smaller than the doses used in these trials."