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poppy seeds can indeed cause a false positive on a drug screen, but only poppy seeds from opium poppy plants, in which morphine is filtered out and then converted into various forms of heroin and other opiates - both opiate and non-opiate poppy seeds are used in cooking, but the non-opiate seeds are more common in products for the consumer. those little seeds you see on a bagel or bun are non-opiate seeds.
additionally when the test are conducted there and there is a presence of opiates an additional test is conducted to protect against false positives and it can be seen on the test results. what they do is test for "thebaine" which is a natural constituent of poppy seeds that does not show up in the final drug products from opium poppy plants.
in essence, if a urine sample that tests positive for opiates also tests positive for thebaine, the urine opiates were from eating poppy seeds, not drug use. this is how they know if it is indeed a false positive - no one wants to make it tough for someone who is not using so that safeguard is build into the testing procedures.
so what your son will want to do is look at those results and make sure they did that additional test for the "thebaine".