[marianco]

A 5.5 ml injection of T doesn't sound like HRT unless it is a once a month injection.

21 gauge needles cause a lot of tissue damage and hurt a lot and contributes to a lot of scar tissue formation from repeated injections. It is useful for torture.

23 gauge needles can hurt for weeks. It is useful for drawing before switching to a smaller gauge for injection.

25 gauge is useful to minimize pain on intramuscular injections. It isn't so small that it takes too long time to inject a thick liquid like T.

27 gauge and smaller are useful for subcutaneous injections.

With a smaller gauge needle, the problem with intramuscular injections is keeping the needle still while injecting T at a slower rate. Sideways movements of the needle causes tears in the muscle. 25-gauge is a good compromise compared to 27 and smaller gauges.

With smaller gauges than 27, the problem with subcutaneous injections is that the thinner needles more easily bend and thus may have difficulty in penetrating the skin of some patients which are thicker than others. Thus a compromise size like 27-gauge is chosen - a size small enough yet a needle which is stiff enough for injection.