This is the 2007 version. Click here for the 2017 chapter 03 table of contents.

Effects of Marijuana

First-time users of marijuana often feel nothing unless they take a very potent form of the drug. Second- or third-time users may experience profound changes in consciousness. Colors may seem brighter, music more vivid, humor more hilarious. Many experience a desire to eat. This well-known association of cannabis consumption with eating has inspired scientists to look for compounds that curb appetite by blocking cannabinoid receptors in the brain. A team of 10 researchers from the Psychology Department at the University of Connecticut found that cannabinoid receptor antagonists (substances that block or oppose the action of cannanoid receptors) did indeed reduce food consumption by laboratory rats (McLaughlin, Winston, Swezey, Wisniecki, Aberman, Tardif, Betz, Ishiwari, Makriyannis and Salamone, 2003).

What are common effects of marijuana? How did cannabinoid antagonists affect rats?

Research on rats and mice suggests an "anti-aggressive effect" of marijuana (Miczek, 1978). Violence among marijuana smokers is rare unless it is combined with other drugs like alcohol or cocaine. The famous La Guardia commission report from the 1930s noted with surprise that Harlem residents who smoked "reefers" were likely to be found on the roofs of buildings, commenting on the beauties of the night sky, rather than committing crimes.

In 1969 researchers at the National Institutes of Mental Health felt it was necessary to seek a "long-term, multi-disciplinary study of chronic marijuana smokers who weren't taking a lot of other drugs." The result was a two-year study involving more than 2,000 regular marijuana users in Jamaica. Marijuana has been used for 100 years in Jamaica, and nearly 60% of the rural, working-class males smoke it, usually mixed with tobacco. Jamaican ganja (marijuana) is a powerful variety. Women and children also use the drug, but typically they take it in the form of teas and tonics.

What were the findings of the "Jamaica study"?

In the Jamaica study, 30 male ganja users were matched with 30 non-users of the same age. Men in the ganja-using group averaged seven pounds lighter and tended to have bloodshot eyes. Other than that, few differences turned up in biological or intellectual tests. Testosterone levels in the two groups were identical. The only medical difference was a tendency to hyposia (reduced oxygen delivery to tissues) in the ganja-using group, probably due to the carbon monoxide present in smoke. Research with long-term marijuana users in Costa Rica produced similar conclusions.

Studies by Robert Block and colleagues in the early 1990s suggested that chronic marijuana use could have damaging effects on cognitive abilities (Block & Ghoneim, 1993) Similar findings were reported by Pope and Yurgelun-Todd (1996). Block said these findings justified serious concern, but he noted, "There is far more extensive, consistent evidence of cognitive deficits associated with heavy use of alcohol relative to marijuana" (Block, 1996, p.560).

A discussion of the issue by Block, written as an editorial for the Journal of the American Medical Association (JAMA) is at this location:


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