Propstop is absolutely correct with his false positives.
There is a particular cheese which causes positives to Amphetamines.
Poppy seeds can cause a positive to Opiates as does any Codeine containing medication.
I am aware of a mine site employee who popped a Panadine Forte (prescribed) any time he saw the drug testers. The preliminary positive had him stood down from work on full pay till the laboratory test confirmed it was prescribed, permitted medication. That could take between one and four days depending. He got paid to relax at the mine camp until cleared.
I seem to remember Victoria Police commenced Random Drug testing with a large media contingent to see the first "Drug Driver" arrested.He had his Photo on the front page and on each of the TV News channels. The guy was in a company vehicle and was sacked before the results came back, NO DRUG DETECTED. A false positive.
The Road side test is VERY unreliable.