Know Your Needles A Quick Nursing Guide! , Today, lets understand the different types of needles and their clinical purposes , Green Needle (21G) For IM injections like vaccines or If youre still guessing needle sizes were not doing that in 2026 , Heres what you MUST know:, Smaller gauge number = BIGGER needle, 1820G = blood & trauma, 2123G = IM, 2527G = For subcutaneous injections, using shorter needles (4 5mm) is the safest choice to ensure medication stays in the fatty layer and avoids muscle injury. A 4mm needle is ideal, minimizing pain while keeping B12 Injection Needle Size B12 Syringes Is b12 im or sq