Drug testing poses a major potential problem for the hemp food industry. In 1996 an employee who had eaten a Seedy Sweetie snack failed a drug test for marijuana. The candy is made by Hungry Bear Hemp Foods using pressed hempseed. Normally it does not contain THC, but apparently a detectable amount of residue from leaves slipped through the cleaning process. Aegis Laboratories found positive readings in one person’s urine sixty hours after consuming the candy, and similar cases have arisen in other states. The Department of Transportation issued a policy guide to "never accept an assertion of consumption of a hemp food product as a basis for verifying a marijuana negative. Whatever else it may be, consuming a hemp food product is not a legitimate medical explanation for a prohibited substance or metabolite in an individual’s specimen. Rather than recognizing the inherent flaw in its testing system, the DEA is instead considering making hempseed snack bars illegal, since they may occasional trigger false positives.

(Taken from Hemp For Health - The Medicinal and Nutritional Uses of Cannabis Sativa by Chris Conrad.)