Headache
If you experience a headache when using vitamin B12, you should not panic. Some people who use high doses of vitamin B12 report experiencing headaches [6]. The headache is typically not severe and does not interfere with a person’s ability to perform daily tasks. Once your body adjusts to this vitamin, the headache will disappear. If you have a headache that does not go away while taking vitamin B12 supplements, you should see a doctor. This is not normal.
Nausea and Vomiting
Nausea is a sick feeling that heightens a person’s urge to vomit. It may or may not lead to vomiting. You may feel nauseated or vomit when using vitamin B12. This is usually associated with taking high doses [7] of the vitamin, such as those recommended for people with pernicious anemia. Pernicious anemia is a chronic and progressive anemia that affects adults. It is believed to be caused by an inability to properly absorb vitamin B12.
If you have nausea or vomiting without any known cause when taking vitamin B12, you should see a doctor immediately. While it is sometimes reported as a side effect of vitamin B12, it is not normal [8]. A doctor can try to find out why you are experiencing this symptom.