None of my employers have ever collected DNA. Nearly all of the companies I have worked for domestically and internationally practiced pre-employment drug screening, annual screening, random, and reasonable suspicion testing.   This necessitates having employees list the meds that they are on since if a test is failed and a substance is not prior-ly disclosed.... termination is immediate.  Refusal to test is also immediate termination.

Dangerous industrial environments are dangerous enough without people participating in enjoying liquid, narcotic, or pharmaceutical dumb ass while working.  The safest substance to use when not at work is alcohol since it is not discernible in tests after 24 hours. Even when someone smells like a distillery if it has been 24hrs they will blow a 0.00 in a breathalyzer test.  I had a direct report supervisor on my staff who came to work smelling like he had been swimming in vodka.  A Sr. client manager demanded we terminate him.  I got a call from the plant manager, I updated him on the investigation and the 0.00 BA test, and I informed him I had sent the employee to the showers and to change clothes. The plant manager had my back and we did not have to fire the supervisor. Though that individual was under a hairy eyeball for quite a while after that.

I have had to fire people for refusing reasonable suspicion tests when they were witnessed and reported for having some dark corner transactions including money and small glassine packets.  They refused to provide a urine sample, they were gone within an hour.

Some industries cannot tolerate people being under the influence of any intoxicating substances.

Partaking of various substances if someone is a 100% home office coder, etc... no problem.  Being baked, or loaded, or tweaking during work in those types of professions does not represent a safety or health risk to others.

My own testing history includes a very late faint negative indicator for Cocaine. Every drug test I have ever had to take, Cocaine looks like it will be + and very near the time limit for the results to show I will get a faint line under Cocaine.  Those tests show a line for negative.  Any line, is a good line. Even when it is exceptionally faint.

I am on a ton of Rx meds and have been since I was in my mid teens.  I take no chances. I list them all when I am selected for testing. Even as the Sr. leader I have always kept myself listed in the random selection software with my people.  One year, I was selected 9mos in a row by the random number generator.  My team would give me loads of shit for that.  Because of that, when someone would complain about their number coming up a few times they would immediately get pummeled by my team that  made it clear that "The boss gets pulled all of the time. Quit whining!"