More specifically, the methylcobalamin form of B12 is recommended, as it has been shown to be the most effective. Taking B12 gives you a huge boost of energy while training, and more importantly, greatly helps your recovery.
First things first. Sterility. Only use new pins and do not share, do not let the pins make contact with anything that may be hazardous by keeping the plastic covering on when not being used, use substances which you are sure are legit, reputable, and are sterile, wash hands thoroughly before any procedures, inject in a clean sterile environment, and make sure the injection site is cleaned with antibacterial agents before and after administration. It’s easy to just skip this paragraph or become lazy over time, thinking that you will be fine. If you want to reduce your risk of infections (which can be very painful and put you out of training for a while), then always think about sterility before and after administration. Be smart.