indicate, but they seem to support the general contention that marijuana impairs thinking.

In the 1970s, reports suggested that heavy marijuana use causes structural changes in the brain, but this finding has not been confirmed when examined with more sophisticated techniques. While more recent studies have found that heavy marijuana users make subtle mistakes in cognitive tasks after they abstain from the drug for 19 to 24 hours, some researchers have questioned the validity of this conclusion because the users may not have been matched against nonusers with comparable cognitive abilities.28

Marijuana has also been shown to affect activities that require a fine balance of attention and muscular coordination, such as driving. Such functions are governed by psychomotor processes, which include the ability to control body and limb movement, sustain attention, and respond to environmental cues with appropriate movements. A study of experienced airplane pilots showed that their performance on flight simulator tests was impaired as long as 24 hours after smoking a single marijuana cigarette. Interestingly, prior to taking the test the pilots told investigators that they were sure their performance would not be affected.29

Clearly, the evidence that marijuana impairs cognitive and psychomotor performance indicates that medical users will need to limit their activities—much as after taking a strong painkiller or drinking alcohol. No one under the influence of marijuana or THC should drive a vehicle or operate potentially dangerous equipment.

One of the most controversial effects that marijuana has been claimed to produce is a so-called amotivational syndrome. Although this syndrome is not a medical diagnosis, it has been used to describe the behavior of young people who lose interest in school, work, and social activities. When heavy marijuana use accompanies this behavior, the drug is often cited as the cause, despite the fact that no convincing data demonstrate that marijuana actually provokes these symptoms.

It is not enough to observe that chronic marijuana users lack drive or ambition. In order to justify such a claim, people's behavior and personality traits must be compared before and after they become regular users. Because it would be unethical to en-

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