Diarrhea
High doses of vitamin B12 may cause diarrhea [9]. Most people who take vitamin B12 supplements do not experience any side effects, including diarrhea. If it happens to you though, it should not be a problem. If the diarrhea is serious or lasts long, you need to check in with a doctor. It may be that an underlying health condition or a stomach bug is causing this symptom.
Fatigue or Weakness
Some people take high doses of vitamin B12 and experience symptoms such as mild fatigue and weakness [10]. While this is unusual, it is not a cause for alarm, unless your symptoms are severe or persist for days. Except you have pernicious anemia or severe vitamin B12 deficiency, there is no reason to take more than the recommended daily allowance of vitamin B12. There is no evidence that taking high amounts of vitamin B12 makes it more effective in improving a person’s body function.