Vitamin B12 is a relatively safe substance. However, some people may be more likely to experience problems with it than others. You should talk with your healthcare provider prior to taking this vitamin if you have:
- Had a coronary stent
- Cobalamin or cobalt sensitivity
- Leber's disease
- Polycythemia vera
- Severe megaloblastic anemia
- Any allergies, including allergies to foods, dyes, or preservatives.
Also, let your healthcare provider know if you are:
- Pregnant or thinking of becoming pregnant
You should also be sure to tell your healthcare provider about all other medicines you are taking, including prescription and non-prescription medicines, vitamins, and herbal supplements.
Specific Vitamin B12 Warnings and Precautions
Warnings and precautions to be aware of concerning the safety of vitamin B12 include the following:
- Preliminary evidence suggests that vitamin B12 (in combination with folic acid and vitamin B6) may actually do more harm than good when used after coronary stenting. This combination of supplements may actually increase the risk of re-narrowing ("restenosis") of the coronary artery.
- People with cobalamin or cobalt sensitivity or allergy should not take vitamin B12, since it contains the metal cobalt.
- People with early Leber's disease (an eye disorder) should not take vitamin B12, as severe damage to the optic nerve could result.
- Vitamin B12 may unmask symptoms of polycythemia vera (a disease involving too many red blood cells). This vitamin does not cause this disease, but it can make the symptoms more noticeable.
- Treating severe megaloblastic anemia with vitamin B12 too aggressively may result in dangerously low blood potassium (hypokalemia) and gout in certain people. Hypokalemia has the potential to become fatal if left untreated.
- Folic acid can "mask" a vitamin B12 deficiency. Although folic acid can correct anemia due to a deficiency, making it more difficult to detect and diagnose, folic acid does not prevent the nerve damage that a vitamin B12 deficiency can cause.
- Vitamin B12 can interact with some medications (see Vitamin B12 Drug Interactions).
- Normal intakes of vitamin B12 are safe for pregnant or breastfeeding women (see Vitamin B12 and Pregnancy and Vitamin B12 and Breastfeeding).