For the first dose of intramuscular vitamin B-12, which needle length is recommended?

• A. One inch
• B. One to one and a half inches
• C. Two inches
• D. Three quarters of an inch

Answer: (B)

Delivering vitamin B-12 by intramuscular injection works best when the needle reaches muscle tissue, not subcutaneous fat. The depth needed to deposit the medicine into muscle varies with body size, so a length that reliably gets into muscle across a typical adult range is preferred. One to one-and-a-half inches is a common choice because it accommodates people with average to somewhat higher body fat, helping ensure the injection lands in muscle rather than fat. A shorter needle (like three-quarters of an inch) can end up subcutaneous, leading to slower, less predictable absorption. A longer needle (around two inches) is usually more than necessary for most adults and can cause more discomfort or risk, though very obese patients might sometimes require more depth. So, the recommended range balances reliable intramuscular delivery with patient comfort for the first dose.