Not every one has the same symptoms of vitamin B12 deficiency. You and your best friend may both be low in vitamin B12, but have very different symptoms. Because the symptoms of low vitamin B12 vary so greatly the medical community has often concentrated on one symptom and ignored others. The result is that many people with severe symptoms of low B12 do not get treatment because they don’t have the symptom popular on a given day, the “symptom d’jour” you might say.
Low B12 & Heart Palpitations - "Heart palpitations are one symptom of anemia, one form of which is caused by low vitamin B12. According to Elizabeth Crow in the book "Prevention's Ultimate Guide to Women's Health and Wellness," a decrease in red blood cells may come from vitamin B12 deficiency, which can lead to weakness, headaches, fatigue and heart palpitations. Strict vegetarians may be the most susceptible to this form of anemia, as vitamin B12 is only found in animal foods."
New research showed nearly 20 percent of those taking metformin had borderline low vitamin B12 levels which lead to vitamin B12 deficiency. http://articles.mercola.com/sites/articles/archive/2016/03/30/vitamin-b12-deficiency-metformin.aspx