When I take 5mg methylcobalamin it causes me tachycardia. I then need massive amounts of potassium to calm down my heart rate (about 10 grams). Why? I also take 3 x 800mg methylfolate. When I take it with hydroxycobalamin, I can control my POTS/tachycardia with a steady dose of about 5 grams potassium daily. When I switch to methylcobalamin, my heart rate gets out of control and I need much more. I have heard the theory that there is a drop in potassium levels, caused by new cells being created. But come on, 10 grams!? Also, I do have doubts about this theory. What about other people who create lots of new cells, like people who just gave blood, people who had surgery, people who had chemotherapy, or athletes/bodybuilders? I have never heard of anyone needing to take 10 grams of potassium daily. Thank you for any comments.