How does a home drug test detect drugs in your urine such as meth and why do some prescription drugs test positive?

Drug tests. Drug tests are used as much to determine what the person has not taken as to what the person has taken. Drug tests may look for illegal drugs. More often they test patients to make sure that they are taking their medications. Sometimes patients sell their medications or feel that they do not need the medication and stop taking it. Drug tests are used for these purposes.