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Exposing Marijuana Myths:
A Review of the Scientific Evidence

From The Lindesmith Center
A Project of the Open Society Institute
Ethan Nadelmann, Director

Lynn Zimmer, Associate Professor of Sociology, Queens College
John P. Morgan, Professor of Pharmacology, City University of New York Medical School

October 1995
Open Society Institute/The Lindesmith Center


Since the 1920s, supporters of marijuana prohibition have exaggerated the drug's dangers. In different eras, different claims have gained prominence, but few have ever been abandoned. Indeed, many of the "reefer madness" tales that were used to generate support for early anti-marijuana laws continue to appear in government and media reports today.

For a while in the 1970s, it seemed as if scientific inquiries were beginning to influence the government's marijuana policies. Following thorough reviews of the existing evidence by scholars 1 and official commissions, 2 criminal penalties for marijuana offenses were lessened and a number of states moved in the direction of decriminalization.3 However, in response to lingering concerns about marijuana's potential toxicity, the government expanded its funding of scientific research, mostly through the newly-created National Institute on Drug Abuse (NIDA).

Probably the most important studies of the 1970s were three large "field studies" in Greece, Costa Rica and Jamaica. These studies, which evaluated the impact of marijuana on users in their natural environments, were supplemented by clinical examinations and laboratory experiments oriented toward answering the questions about marijuana that continued to be debated in the scientific literature. The data from these studies, published in numerous books and scholarly journals, covered such matters as marijuana's effects on the brain, lungs, immune and reproductive systems, its impact on personality, development, and motivational states, and its addictive potential.4

Although these studies did not answer all remaining questions about marijuana toxicity, they generally supported the idea that marijuana was a relatively safe drug -- not totally free from potential harm, but unlikely to create serious harm for most individual users or society. In the years since, thousands of additional studies have been conducted, many of them funded by NIDA, and together they reaffirm marijuana's substantial margin of safety. Our review of that body of work reveals an occasional study indicating greater toxicity than previously thought. But in nearly all such cases, the methodologies were seriously flawed and the findings could not be replicated by other researchers.

Especially since the 1980s, when the federal government's renewed war on cannabis began, both the funding of marijuana research and the dissemination of its findings have been highly politicized. Indeed, NIDA's role seems to have become one of service to the War on Drugs. Dozens of claims of toxicity appear in its documents, despite the existence of scores of scientific studies refuting their validity. At the same time, studies that fail to find serious toxicity are ignored.

In the following pages, we review the scientific evidence surrounding the most prominent of the anti-marijuana claims.