There is some evidence that THC metabolites can be detected in heavy, chronic marijuana users for several weeks after their last use.
For more detailed information about drug testing for traces of marijuana, see How Long Does Marijuana Stay in the Body?
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National Institute on Drug Abuse. "Marijuana." DrugFacts Updated January 2014
National Institute on Drug Abuse. "Want to Know More?- Some FAQs about Marijuana." Marijuana: Facts for Teens Updated October 2013
National Institute on Drug Abuse. "Marijuana." Research Report Series Updated July 2012
The Partnership at DrugFree.org. "Marijuana." Drug Guide. Accessed April 2014.