Can B12 injections cause irregular heavy menstrual bleeding?

NO. Vitamin B12 is extremely safe and has no known toxicity. There is no known association with menstruation. See http://www.Crnusa.Org/safetypdfs/013crnsafetyvitaminb12.Pdf if you have heavy bleeding B12 (along with iron & folic acid) will help you make new blood cells. Irregular heavy bleeding may be caused by fibroid tumors or other imbalances and should be evaluated by a gynecologist.