while vitamin b12 is safe for most people some individuals may experience side effects such as diarrhea and blood clots. for more about the side effects see this webmd piece. the only way to know for sure is to see a doctor. don't delay getting help!
does b12 (2500 mcg) causes bloody stool? . i just started taking sublingual vitamin b12 (methylcobalamin) and noticed that my stool is bloody. is this a normal reaction to the vitamin?