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Vitamin B12 overdose is extremely rare, and virtually no examples have been recorded. For this reason, almost no symptoms of a vitamin B12 overdose have been reported. Possible symptoms could include diarrhea or a moderate allergic reaction, including itching or rash. While overdose is almost never seen, it's possible that an allergic reaction could occur to shots containing B12.
The recommended daily allowance (RDA) for vitamin B12 is 2.4 micrograms (mcg) for most adults and somewhat higher for pregnant and nursing women. Vitamin B12 has been found to be safe in healthy people taking 1,000 mcg daily over periods of years. Much higher amounts are still unlikely to cause a vitamin B12 overdose. This is probably because extra B12 is quickly excreted by the kidneys or because it is no longer absorbed at all over a certain amount. People with a B12 deficiency or other health problems may wish to consult a doctor before taking B12 supplements.
Vitamin B12 is sometimes prescribed to treat the condition of pernicious anemia, which usually includes a serious B12 deficiency. In these cases, vitamin B12 may be given in the form of extended release tablets or by injection. Injection is more common in people who have a decreased ability to absorb B12 when it is taken orally. A vitamin B12 overdose in such cases is unlikely because the doctor will determine the correct dosage.
It's possible that an allergic reaction could occur in response to shots containing B12. It's not clear, in the few such known cases, whether the reaction occurred to the vitamin itself or to something else in the shot. It's possible the allergic reactions occurred in response to preservatives. As these cases didn't include a vitamin B12 overdose, the symptoms would be those of a severe allergic reaction. Severe allergic reactions are medical emergencies because symptoms include difficulty breathing and swelling of the throat, face, or tongue.
While overdose is rare, B12 deficiency can occur in people who don't eat meat or the elderly. It's thought that older people don't make enough of the substance the body needs to absorb B12. B12 deficiency may be fairly common in individuals more than 60 years old.
It's still important to keep vitamins out of the reach of children, even if B12 is fairly safe. In the United States, iron poisoning is one of the leading causes of poisoning in young children. Iron has a widespread presence in supplements and multivitamins.