How can I prove that I dont do drugs?

Urine test was positive on several occations, I only take Prescription Medication, after an open heart surgery. Doctors and chemists are denying it is possible. Pathologist says could happen to show false positive.
I am frustrated as I lost two jobs over this issue. I am 65 and can't afford the embarassement. I need to work for 2 more years.

You don't provide any information on what prescriptions you take or what illegal drugs they are detecting. If you're in the USA, the agency that sets the standards for federal drug testing has a helpline (info below). They should be able to answer your questions. It will help if you have specific information when you communicate with them including the name of the labs that did the tests.

Collect all of the information you can on the subject then take it to a lawyer; a labor law specialist if you can find one.

HHS Substance Abuse and Mental Health Services Administration
1-800-Workplace, 1-800-967-5752
HELPLINE@SAMHSA.HHS.GOV