Usually doctors use urine analysis to help diagnose certain infections, kidney problems, diabetes, dehydration and the like. They don't typically screen patients for drugs, unless the patient has been informed of the test in advance.

There is no guarantee that all doctors will follow these guidelines, though. A doctor might be more inclined to test a teen for drug use, if a parent asks him or her to do so. You might want to ask your doctor about her or his confidentially policy to be sure.

As for the amount of time a drug stays in the system varies, Dr. Karen Rosewater says it depends on the drug and how frequently it is ingested. She did note that marijuana tends to stay in the body a lot longer than many other drugs do, however.

For more information, you might want to check out this FYI on marijuana.

Take care.