I take, and love, methyl b12 and adenosyl b12 and also 5-MTHF. I was recently warned by someone that taking too much can cause cancer. I did a small amount of research, basically googling "b12 cancer" and outside the first few saying it reduces quite a few cancers, I found: http://www.usingvitaminswisely.com/vitamin-b12-overdose.html Mainly this part: The link between high levels of B12 and cancer is easy to explain. Vitamin B12 stimulates cell division and it doesn’t distinguish between healthy and unhealthy cells. The excess vitamin B12 provides the extra energy needed by cancer cells. Troubling B12 cancer connections: One case was reported of leukemia resulting from a vitamin B12 megadose used in the treatment pernicious anemia. http://www.amjmed.com/article/0002-9343(84)90872-6/abstract High levels of folate and B12 are associated with an increased risk of prostate cancer. http://psa-rising.com/eatingwell/vb12_folate04.html Diets high in cholesterol, animal protein and vitamin B12 have been linked to certain kinds of stomach and esophagus cancers. http://www.sciencedaily.com/releases/2001/10/011030225654.htm http://www.ynhh.org/healthlink/cancer/cancer_12_01.html I also found some study where smokers that took folic acid + b12 had higher incidences of Lung Cancer. Is there some concern with taking too much? How do I know if I'm taking too much? Was the problem with the folic acid study due to using folic acid vs MTHF? Anyone know how to quiet my fears?