Can a plaintiff ask the court to require a drug test from a defendant?

A party can ASK for anything, but it would be a rare case where the Court would order such a test.  The defendant's drug use would have to be relevant to the issues in the case, and there would have to be good reason to believe that the defendant was on drugs.  Perhaps in a child custody case where there was good reason to believe that one parent's drug use was endangering the children?