Couple of types but most commonly used are Cyanocobalamin and Methylcobalamin. Cyanocobalamin offered by most clinics because it’s cheaper but tends to be less effective. Oh yeah, and has trace amounts of cyanide. It’s more expensive (less than a dollar), effective, and cyanide free counterpart is Methylcobalamin. Maybe it’s because I watched too many old spy movies growing up but we only offer Methylcobalamin for that one reason despite the cost to us.