The physician prescribed an intramuscular injection for me due to a lack of vitamin B12. The only time at which I can take such an injection is in the morning. Does this kind of vitamin B12 intramuscular injection invalidate fasting?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Injections that the fasting person takes while fasting are either nutrients that contribute to the building of the body or they are not. If they are of the kind that nourishes the body, then they break one's fast because they have the same effect as food and drink.{C}

If they are not nutrients, then they do not break one's fast because they are not like food or drink nor do they have their (food and drink's) effect.

Accordingly, the respectful questioner should ask his physician about the type of injection that he needs to take. 

Allaah Knows best.