[pom19]

Well, too much of anything is not good.

Vitamin B 12 Side Effects:
http://vitamins.lovetoknow.com/Vitamin_B_12_Side_Effects

Experiencing vitamin B 12 side effects can be rare; however, they do occur. Vitamin B 12 delivered through tablets or sublingual supplements will have fewer potential side effects than vitamin B 12 injections. The following side effects have been noted in both oral supplementation and injections of vitamin B12 unless otherwise noted. The most common side effects of vitamin B 12 supplementation include:

Allergic reactions to preservatives in the preparation for the injection
Mild diarrhea
Anxiety
Panic attacks
Heart palpitations
Insomnia
Breathing problems
Chest pain
Rash and/or hives
Itchy skin
Heartburn
Vomiting
Back pain
Rhinitis (stuffy nose)
In extremely rare cases, the following side effects have been noted:

Congestive heart failure
Pulmonary edema
Peripheral vascular thrombosis (blood clots in the arms and legs)
Folliculitis
Inflammatory acne
Anaphylaxis (most likely related to preservatives used in vitamin B 12 injections)