Oxford University pharmacologist Leslie Iversen reviews the literature on the effects of long-term marijuana use. Iversen finds that most purported cognitive impairment associated with cannabis dissipates when use ceases. While acknowledging there are some likely physical health risks, “there is little evidence that long-term cannabis use causes permanent cognitive impairment, nor is there is any clear cause and effect relationship to explain the psychosocial associations. Overall, by comparison with other drugs used mainly for ‘recreational’ purposes, cannabis could be rated to be a relatively safe drug.”

Leslie Iversen, “Long-term effects of exposure to cannabis,” Current Opinion in Pharmacology 5, issue 1 (February 2005): 69-72.

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