麻豆传媒

DO DECADES of dope-smoking wreck cannabis users鈥� memory and concentration? Or is this just another anti-marijuana myth?

This long-running debate reopened this week with the publication of a US government-funded study which claims that smoking cannabis daily for 20 years or more impairs memory and attention. Its findings are contradicted by others that have revealed no long-term effects.

The latest research involved 102 cannabis smokers in Seattle, Farmington in Connecticut and Miami. Half had smoked for an average of 24 years. The other half, described as 鈥渟hort-term鈥� users, had smoked for 10 years on average. Both groups reported smoking about two joints a day.

In tests such as memorising a list of 15 words, the long-term users recalled 8.5 words on average, 2.5 fewer than both the short-term users and 31 non-users. The long-term users were also slower at mental arithmetic. But in in other tasks, such as sorting cards, they were just as quick.

Both groups of users also tended to over-estimate the time it took them to complete a task, thinking it had taken them a third longer than it really had.

The authors conclude that cannabis has a cognitive effect that lasts beyond the period of intoxication and that the longer you smoke, the worse the effect on memory and attention. 鈥淲e do not know exactly how that translates into real-world problems,鈥� admits team member Robert Stephens, a psychologist at Virginia Polytechnic Institute and State University. Nevertheless, he thinks long-term users might not function as well in day-to-day life.

But there are other explanations for the results. For example, it was only 17 hours on average since the users had last smoked a joint, and some had smoked just 12 hours earlier. Harrison Pope, a psychiatrist at Harvard Medical School in Boston, says other studies, including one he published last year comparing 108 users with 72 non-users, reveal no long-term effects. One difference, he says, is that his subjects were tested 28 days after they had last smoked.

Pope also points out that the subjects in the latest study were all seeking treatment for marijuana dependency, and might have had problems such as anxiety or depression that affected their test scores.

Paul Armentano, director of research for the US National Organization for the Reform of Marijuana Laws, is even less impressed by the results. 鈥淭here just doesn鈥檛 seem to be a large cohort of people suffering these adverse consequences,鈥� he says.

But even if long-term smokers who give up do not suffer any permanent effects, Pope thinks continual heavy use does have an impact. 鈥淲e鈥檝e looked at the lives of these people and how they function. They overwhelmingly rated themselves as less effective and less happy than the control subjects,鈥� he says.