What Causes False Positive For Methanphetamin While On Pain Management Treatment?

Hello, my 72 year old husband goes to pain management and tested positive for methanphetamin. He does not do any type of illegal drugs. How is this possible? He takes prescriptions for low and high blood pressure, plavix, albuterol inhaler, spiriva, If this is not possible for him to test positive when he does not do drugs, what happened and what can we do?