Background
Vitamin B12 is a water-soluable vitamin needed for normal cell activity. It is a part of a group of cobalt containing B complex vitamins, which are also known as cobalamins. Like other B vitamins, vitamin B12is important for metabolism. It also helps in the formation of red blood cells and in maintaining the central nervous system. It is found in most foods which come from animal products including liver, fish, shell fish, meat and dairy products.

Side Effects
The good news is that vitamin B 12 is usually non-toxic, even when taken in large doses. However, people have experienced some adverse effects, even though these side effects are quite rare. Generally, when vitamin B12 supplements are taken orally then there is little chance of side effects occurring. Some people choose to take vitamin B12 administered with an injection. This type of ingestion as been associated with the following side effects:

•mild diarrhoea
•anxiety and panic attacks
•heart palpitations
•insomnia
•breathing problems
•chest pain
•skin rash, hives or itchy swollen skin

Some health professionals believe that patients develop these reactions not because of the B12, but because of the preservatives that are part of the injection formula. Currently, most injections contain preservatives. To be on the safe side, it is probably better to take B12 orally.

Medical Conditions
Other side effects may be experienced by people who have an allergy or a sensitive to cobalamin and cobalt. If this is the case, then it is advisable to avoid vitamin B12 supplementation altogether and instead seek to eat the foods that contain naturally occurring B12. In addition B12 should be avoided by people suffering from Leber’s disease, a hereditary disease in which the optic nerve wastes away. Ingesting vitamin B12 can actually speed up the atrophy of the optic nerve resulting in rapid loss of central vision.

There is also evidence to suggest that patients who have undergone heart surgery and received coronary stents may have an increased risk of the artery narrowing again when they take vitamin B12. A small risk, but one to bear in mind.

On a positive note, pregnant and breastfeeding women can take vitamin B12 quite confidently when it is taken orally in amounts not exceeding the recommended daily allowance (2.6 mcg/day if pregnant and 2.8 mcg/day if breastfeeding)
As can be seen, vitamin B12 is a relatively safe vitamin with very few adverse side effects even when taken in large doses. More problems than not arise from B12 deficiency than they do from ingestion of the vitamin itself.