The reason for using this exact size is not so obvious. There are two types of steroids, oil based (very seldom a thin oil) and water based suspensions. The thick oil based steroids need a thicker needle to suck up the oil into the syringe and to be able to inject it smoothly. This is critical because 99% of all people injection oil based steroid with a needle that is to thin is going to end up with a painful injection site for days.
The reason for this is two fold. The excessive force needed to inject the oil through a thin needle means there is a lot of movement of the needle when injecting and this bruises and damages surrounding tissue. The second issue is that the force with which the oil leaves the needle tip causes further damage to the tissue. Imagine forcing water down a very narrow hose pipe, it will spry out harder and further than out of a big hosepipe.
With water bases suspension, thicker needles are also essential. Because these are suspensions, meaning particles are not dissolved in the water but floats in it so to speak, the risk of these particles clogging the needle is very high. Anybody that has injected Aqua Test for instance will tell you how often it clogs the needle.
This leads to the conclusion that you need a thicker and longer needle to inject deep in the muscle without excessive force. The 21 gauge, long needle is the way to go.
