An overdose occurs when an excessive / dangerous dose of a drug / substance is used. Toxicity would be unlikely with Vitamin B12. Adverse effects from Vit B 12 injections may include: "diarrhea, nausea, headaches, & joint pain"
REF: http://www.vitamins-and-health-facts.com/vitamin-b12-cobalamin-facts-benefits-overdose-deficiency-symptoms.html
