At the Pfeiffer Treatment center they look at a process called methylation. People that are undermethylated get worse when they take folic acid or vitamin B12.

http://www.alternativementalhealth.com/articles/walshMP.htm

"One thing that is absolutely certain is that methionine and/or SAMe usually harm low-histamine (overmethylated persons)..... but are wonderful for high-histamine (undermethylated) persons. The reverse in true for histadelic (undermethylated) persons, who thrive on methionine, SAMe, Ca and Mg..... but get much worse if they take folates & B-12 which can increase methyl trapping."