The best places to administer a vitamin B12 injection are:
Thigh: The outer part of the upper thigh is one of the most common injection sites for vitamin B12 shots because there is enough muscle and fat under the skin to help absorb the B12. The shot should be given right in the middle part of the thigh. To find the right location, visualize an imaginary line dividing the upper leg in half. Place your hand on the line and slide it about half way around the outer part of the leg. Give the injection according to your doctors instruction.
Buttocks: This is the second most common injection site (and less painful than the thigh) and reportedly where Margaret Thatcher preferred her injections.
Arms/Hip: The shoulder muscle (outer deltoid) and the outer hip muscle (the ventrogluteal muscle) can also be used for injection of B12. The hip is a good site for B12 injections because there is less danger of hitting nerves or blood vessels in this area.
Check out a good video below: