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.