Took this from another forum, idk how accurate it is though:
"Pain is caused when the oil/solvents are absorbed by the body and crystals are left behind.
Short esters (propionate or acetate) are harder, more painful crystals with melting points in the 100c range.
A hormone with a longer esters (excluding cypionate - cyp is long but also high melting point) can have a melting point in the 20c-40c range. not far off from human body temp.
Pain is also caused by concentration of your gear.
Building off of point 1: Let's say it takes the body 24 hours to absorb 1mL of a certain oil/solvent blend, and 24 hours to absorb 50mg of Testosterone Propionate. If 50mg (or less) of testosterone propionate is in 1mL of that oil, this injection should be painless. On the other hand, if 100mg of testosterone propionate is in that same 1mL of solution, then after 24 hours the body will have absorbed 50mg and 1mL, leaving 50mg behind in the injection area, crystalized and painful.
Its better to shoot 3mL of 50mg/mL Testosterone Propionate than 1mL of 150mg/mL Testosterone Propionate.
This is also why water based suspensions (Testosterone base/no ester, Winstrol ) hurt the most, water is very easily absorbed in the body
The solvents used can cause pain in 2 ways. Benzyl alcohol (BA) is used at 1%-2% as a preservative and antiseptic. If the alcohol content is too high the gear will burn. Pain in the first 24 hours is usually caused by heavy solvents, pain in the next few hours is usually cause by crystalization. Another way is a bad recipe. if someone used 2% BA, and the rest of the solution oil, the mg/mL would have to be low due to oil's weak ability to hold crystals. On the other hand, a recipe like 2% BA, 5% Guaiacol (super solvent), 10% Benzyl Salicylate (liquid asprin) with the filler split 50:50 between Ethyl Oleate (oil/solvent hybrid) and normal oil should be far less painful.
If you inject too quickly it can tear tissue.
If your muscle is new to the hormone, it will absorb the hormone very slowly, but absorb the oil/solvent very quickly. This will cause more crystalization and pain. As your muscles recognize the hormones, they will be absorbed more quickly, thus less pain. The deeper you inject into the center of a muscle group, the better."