3 Surprising Reasons for a False-Positive Drug Test

Random drug testing is increasingly popular in the work place, and routine urine tests screen for the “big five” illegal drugs: marijuana, cocaine, amphetamines, opiates and PCP. But some legal substances could cause you to fail a routine drug screening and put your job in jeopardy. 

Psychopharmacologist Dr. Eileen DiFrancesco joins The Doctors to explain how these three substances could trigger a false-positive result:

Cough suppressants:

  • Dextromethorphan, one of three ingredients often found in prescription and over-the-counter cough suppressants, can lead to a positive test for PCP, often called Angel Dust. Taken in higher doses, some cough suppressants can show up as an opiate.
  • Pseudoephedrine and ephedra, two other ingredients often found in cough suppressants, can test positive for amphetamines.

Diet pills:

  • Phenylpropanolamine, one of the main ingredients in most diet pills, can cause a positive drug test.
  • Older diet pills that contain ephedra also can lead to a positive drug test.


  • People with chronic pain who use the common over-the-counter painkiller regularly can test positive for THC, the active ingredient in marijuana.

The three substances should be out of your system within 72 hours of taking them. If you get a false-positive result in a drug test, Dr. DiFrancesco says you can request a more comprehensive test.