Random, "amotivational syndrome" is a well known and well documented negative effect from prolonged or frequent use of marijuana.

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Amotivational syndrome, according to researchers, is seen most often in heavy marijuana users. It is defined as "apathy, loss of effectiveness, and diminished capacity or willingness to carry out complex, long-term plans, endure frustration, concentrate for long periods, follow routines, or successfully master new material." Experts also note that verbal communication can be impaired in both speaking and writing, and that some users become more introverted and "totally involved with the present at the expense of future goals." These kinds of symptoms have been described in scientific literature on marijuana use in societies with a long history of the drug, such as Egypt and the Caribbean, as well as in the United States.

</div></font></blockquote><font class="post"> (Source: KQED "You Decide", http://www.kqed.org/topics/news/pers...uana/2yes.html )

That was just to provide a basic definition.