Cannabis and Other Illicit Drugs: Comorbid Use and Abuse/Dependence In Males and Females
Arpana Agrawal et al., "Cannabis and other illicit drugs: comorbid use and abuse/dependence in males and females," Behavior Genetics 34, issue 3 (May 2004): 217-28.
The gateway theory suggests that marijuana use leads to use of hard drugs like heroin and cocaine. People who use marijuana are more likely than those who don't to use other drugs, but that doesn't mean marijuana causes other drug use. This study suggests that underlying genetic and environmental factors encourage use of marijuana and other drugs, rather than marijuana causing hard drug use.
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http://www.springerlink.com/openurl.asp?genre=article&issn=0001-8244&volume=34&issue=3&spage=217
