Understanding Needle Gauges and Injection Sites Needle Size for B12 Injection: 5 Must Know Tips Does size matter? Historically, a bigger needle gauge was considered better to prevent damage to red blood cells. However, our recent guidelines recommend using smaller gauge needles (2325 G) for all 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 = Needle Size for B12 Injection: 5 Must Know Tips Vitamin B12 Injections