Here's something on the topic!
Q: Dude, is a 1-inch needle long enough for injection in to my ass? Oh, and do you have any tips on injecting? I've done it a few times but I'm not even sure if I'm doing it right.
A: A study peformed about 5 or so years ago can provide at least a partial answer to this question. In this study, they used ultrasonography to measure the thickness of the fat pad above the deltoid muscle in 220 adults. What they found was that a 5/8 inch needle would not have allowed enough penetration for a true intramuscular injection in 17% of men and around 50% of the women.
The authors concluded that for men weighing 59-118 kg, or 130-260 lb (rounded), and for women weighing 60-90 kg, it may be better to use a 1-inch or 25mm needle. They also suggested that a woman over 90 kg use a needle 1 1/4 inches long, or 38mm.(15)
So, with these data in mind, I think it's safe to assume that a 1-inch needle is sufficient in all types of men for deltoid injections and the same can likely be said for injections in to the thigh. As for the buttocks, I'd say the safest bet is to go with a 1 1/2 inch needle. However, there are going to be plenty of guys who are lean enough to use a 1-inch in the buttocks and a 5/8 inch in the deltoid, and even a 1/2 inch needle in the thigh. To be 100% sure, however, use the lengths suggested above.
As far as "techniques" that may be useful, one is simply stretching the skin flat while injecting and thus allowing for an easier IM penetration (and this is only after you've thoroughly cleansed and disinfected the area). And, if you wish to keep any of the oil from seeping out just after injection (this would only be a small amount but if you want to be anal), you can use the "Z" track method which is done by simply pulling back on the skin while you inject (pulling with your thumb and fingers over to the side) and then of course, letting go once you've pulled the needle out.
This allows you to create a seal with your skin that will prevent any oil from coming back out just after injecting.
(note) - written by Cy Willson