All drug leaflets list possible side effects of using the drug. Whenever muscle or joint pain is a possible side effect, it is listed in the singular (both muscle and pain), when it is generalized such that no specific muscle or joint is concerned (in many cases it is impossible to predict which muscle or joint will hurt, if any).
So why is it in the singular? Why not "muscles pain" or "muscles pains", which (ostensibly) includes the possibility of several different muscles hurting as a result of the drug?
My mother tongue is Hebrew, and in Hebrew we do say it in the plural, i.e. "muscles pains", Hence the question (I'm not suggesting that Hebrew and English should work the same, just wondering about the English form).