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

unfortunately, often you cannot. this is why there are supposed to be so many laws regulating drug testing, because there is no realistic way to prove that positive results are only coming from perscription medication, and it also shouldn't be any of you employer's business what perscription medications your're taking. often this is the primary reason for drug testing- to get a result that can be used to get more personal information out of a person who suddenly doesn't feel so free to not answer innapropriate questions. most often, the employer doesn't even actually care about proving anything one way or another, and has ulterior motives for calling for a drug test in the first place, i.e. to weed out workers who have chronic conditions by making it look like they are on drugs. i'd say the first step is to get a lawyer.

with a lawyer you can try to sue these companies for violations of privacy and worker's rights, which not only might get you some more money, but it will be the mark on your resume which might fix your reputation. remember: there is never any way to prove that the company is wrong within the systems of the company itself. since violations of privacy and worker's rights, as well as slander, are actual crimes, you are really supposed to go straight to a lawyer and not try to deal with the problem within the corporate system of grievances.