I have rapid weight gain, heart palpitations, swelling and all b12 toxicity symptoms. i tested and my b12 is abnormally high, leukemia isin my genes?

Possibly: It is possible that cyano-cobalamine might be the cause of symptoms since it releases cyanide but methylcobalamine the preferred form, should not cause side effects. If stopping the b-12 supplement gets rid of your symptoms that would be evidence that the supplement is the culprit. Possibly you need other b vitamins for b-12 to work properly like methyl folate (folic acid) and b-6. Look at your life style.