You should speak with your head nurse and ordering physician, because if it was ordered sc and you gave it IM that's a med error (wrong route). However, we give Vit B12 frequently IM, never sc. Still, you need to clarify.
at the facility where i work we administer b-12 either way im or sq. however, if it's ordered for a specific route with we follow the order, take your time and make it a habit to double check your orders. therefore, i must side with joanna73, this is considered "med.error", don't despair the worse that could happen is a pet-talk from your nm after you inform the mishap. lastly, learn from the experience and move on...wishing you the best always ...aloha~