Drug screening is a common practice in many companies today. Giving a drug test before an employee is hired lets the company know if the employee has any issues with drugs. In jobs that require concentration or a high level of responsibility, it's important to have drug screens, especially if one's actions can lead to hurting or even killing someone. I personally wouldn't want a crane operator working for my company who is high on drugs and putting every person around them in danger.
Having random drug tests at work let's employees know the importance of staying off drugs because at any time they could be tested. Sometimes an employee may be slacking at work. They may be showing signs of drug use and the employer may not want to fire the individual. Some companies have programs that provide treatment if an employee fails a drug test. Sending the worker to a rehabilitation center that they believe can get the worker clean and back to work.
Most companies on the other hand have a zero tolerance policy. Zero tolerance means that if anyone fails a drug screening for any illegal drug or prescription medication without a legitimate prescription, they will be terminated immediately. Failing a drug test can ruin one's career and lead to a domino effect of life-changing circumstances.
So what happens if someone fails a drug test but never used any drugs? The drug tests used today may be much better and cheaper than the ones in the past, but they are still not infallible and sometimes produce false positives. In fact, innocently eating a bagel or muffin with poppy seeds can make a drug free individual look like a heroin addict on a drug test. Opioids can be detected in urine for atleast 48 hours after one eats food containing poppy seeds. As little as a single bagel covered with poppy seeds could produce a false positive test for opiates. What most people don't realize is that opiates/opium are made from the poppy plant. Though you won't get "high" from eating a poppy seed bagel, it could easily register as a false positive for an opiate test.
In 1990 a Saint Louis police officer was suspended for 4 months because he failed a drug test for opiates after he had eaten four poppy seed bagels the day before a drug test. He was reinstated with back pay after it was determined that poppy seeds caused his false positive. His case was especially strange for his department because the officer had a clean work record and had no indications of any problems before his random drug screening. The department, curious as to his false positive decided to try an experiment. They had another police officer eat 4 poppy seed bagels and then take a drug test. He too tested positive for morphine/opiate use confirming confirming the poppy seed effect theory.