At this point, I would say basically everyone -- or at least the majority of people -- have been drug tested at some point in their lives. Whether this is was for a high school/college sport or for a job, drug tests are pretty common. Whenever word about an athletic drug test got out at my high school, a running joke among students was that all athletes should stay away from poppy seeds for a while in order to avoid their drug test producing a false negative.
As a former athlete myself, I was no stranger to the poppy seed joke, but I never fully understood why or how this random thing showed up in a test that was searching for drugs. It turns out that a handful of opiates -- codeine, morphine, and heroin -- are actually produced from the seeds of poppy flowers. All three of these opiates can be (and typically are) abused by some people. Therefore, ingesting poppy seeds can almost immediately lead to traces of opiates in the system, which can then be picked up by drug tests which are administered either in schools or in places of employment.
Poppy seeds are totally legal to be used in foods in the United States, but consumers should be aware of the repercussions of ingesting this ingredient. People who fall victim to a false negative drug test due to poppy seeds being in their system can opt for a more specific test (usually done on a hair follicle or through a blood sample) to prove that their system is clean of actual drugs.