Increased cancer incidence could be a potential side-effect from supplementation, because folate is involved in purine and pyrimidine synthesis. What do controlled trials say?
Simply put: no risk, no benefit in recent analyses (1, 2). An earlier (now outdated?) meta-analysis suggested increased risk of cancer (3). Perhaps some time I can read the full papers:
"Our analyses suggest that cancer incidences were higher in the folic acid-supplemented groups than the non-folic acid-supplemented groups (relative risk=1.21 [95% confidence interval: 1.05-1.39]). Folic acid-supplementation trials should be performed with careful monitoring of cancer incidence" (3)
Almost the same holds true for CVD (4). Folic acid's useless.
1. Lancet. 2013 Mar 23;381(9871):1029-36.
Effects of folic acid supplementation on overall and site-specific cancer incidence during the randomised trials: meta-analyses of data on 50,000 individuals.
Vollset et al.
Int J Cancer. 2013 Sep 1;133(5):1033-41. doi: 10.1002/ijc.28038. Epub 2013 Feb 15.
Folic acid supplementation and cancer risk: a meta-analysis of randomized controlled trials.
Qin X, Cui Y, Shen L, Sun N, Zhang Y, Li J, Xu X, Wang B, Xu X, Huo Y, Wang X.
Cancer Epidemiol. 2012 Feb;36(1):78-81. doi: 10.1016/j.canep.2011.05.003. Epub 2011 Oct 21.
Meta-analysis of cancer risk in folic acid supplementation trials.
Baggott JE, Oster RA, Tamura T.
4. Eur J Intern Med. 2012 Dec;23(8):745-54. doi: 10.1016/j.ejim.2012.07.004. Epub 2012 Aug 11.
Efficacy of folic acid supplementation in cardiovascular disease prevention: an updated meta-analysis of randomized controlled trials.
Yang HT, Lee M, Hong KS, Ovbiagele B, Saver JL.