Depends on the form of injectable. Methylcyanocobalamin will put stress on the liver if you happen to drink heavily, have liver issues, or use oral steroids; This is because it has been methylated to increase the half life to a week. Cyanocobalamin is the safe one, non-methylated, but it has a half life of about a day thus requires daily injections. It actually has a cyanide group but the amount is so negligible there is no toxicity issue.
Story time:
A few years ago I used to get my heroin from a pretty big time dealer. I knew the guy on personal level and he got a kick out of me because I called him "Boss". Anyways, his heroin was always potent as fuck. However, it was was always cut with B12 (Cyanocobalamin) because it happens to be water soluble and rather safe, cheap, and unsuspicious cut. I was doing his heroin daily for years and during this time I used to have an incredible amount of energy and I felt great on account of all the B12. This was the healthiest dope in New York lol.