it has been shown that most people who have problems with drugs much more severe than marijuana (cocaine, heroin, etc.) started with marijuana, and when the high of marijuana (or the lure of making money from selling it) wasn't good enough for them anymore they moved on to bigger and more dangerous things. Not sure if this was covered or not, but this is actually untrue. The vast majority of marijuana users exclusively use marijuana. Alcohol is a far more frequent gateway drug. Also, you can test positive for using marijuana as much as a month after smoking it, and possibly even longer. Other interesting notes, one third of the US population has smoked marijuana. 20 million smoke it a year. 2 million smoke it every day. Almost 100% of all marijuana sold in the US is grown in the Midwest, and it is estimated to be the largest cash crop in the US by a wide margin (estimated worth of $25 billion, compared to $19 billion for corn. For further comparison, estimated worth of apples? $1 billion.). Marijuana is also one of the few things that you can be tried simultaneously at local, state and federal level, meaning you can get three separate convictions for one incident. Also, 700,000 people are arrested every year for marijuana possession. That's more then any other drug. All these facts and more can be found in the trio of essays called Reefer Madness by Eric Schlosser (author of Fast Food Nation). I highly recommend reading it.