There is mounting evidence that high levels of folic acid can increase cancer risks. Prior studies had shown prostate cancer risks increasing and now a study suggests that breast cancer risks may be higher if your folate intake is high. This is troublesome since it was done in pre-menopausal women. This becomes very problematic where patients have certain genetic issues like MTHFR polymorphisms that require higher folate intake. If you are trying to get pregnant, they recommend a minimum of 800mcg, but usually people take 1mg. So if you are trying to get pregnant, more folate is not necessarily better.
 
BMC Cancer. 2009 Apr 24;9(1):122