My 21-y/o daughter is plagued by daily headaches with all test results normal. any suggestions as to cause?

Daily headaches: A frequent cause are migraine headaches and or tension headaches. Your daughter needs to see her doctor for an examination to determine the cause.

Taking meds?: Surprisingly, regular use of analgesics like tylenol (acetaminophen) or advil can be a cause of headaches. When someone with chronic headaches has a negative workup, it can be helpful to have a trial off the over-the-counter headache medicines. It may be worthwhile discussing this with the physician working up the headaches.