Cannabis use 'dulls the brain'

The study tested 102 heavy users of the drug

Long term marijuana users may have worse memories and poorer attention spans than other users of the drug, scientists say.

Memory and attention span got "significantly" worse the longer a user had been taking the drug, according to tests done on those entering a US drugs treatment programme.

But it is not clear whether giving up the drug will enable users to recover, and the research fuels the scientific debate over the true impact of marijuana use on the brain.

The research - published in The Journal of the American Medical Association (Jama) - shows habitual users may suffer academically, at work and in their interpersonal relationships, its authors say.

Impaired learning

The study analysed near daily cannabis users; 51 long-term users, 51 shorter-term users, and 33 non-users.

The tests to assess attention, memory, and other brain functions were done after an average abstinence of 17 hours.

• There were clear differences between those who had used the drug for 24 years, and those who had taken it for a decade.

• In verbal learning tests, long-term users recalled fewer words than shorter-term users or controls.

• Long-term users showed impaired learning, retention, and retrieval compared with controls and both user groups performed poorly on a time estimation task.

The authors said performance "correlated significantly with the duration of cannabis use, being worse with increasing years of use".

Can brain recover?

The research team was led by Nadia Solowij, of the University of New South Wales, Australia, with colleagues with the Marijuana Treatment Project Research Group - and the study was done in the US between 1997 and 2000.

But Dr Harrison Pope, of the Biological Psychiatry Laboratory, at McLean Hospital, Harvard Medical School, said other recent studies had shown only limited "cognitive" impairment among long term users.

"Even if lifetime duration of cannabis use is associated with greater impairment after 17 hours of abstinence, the data are insufficient to know whether greater impairment would be present a week or a month later," he writes in the same issue of Jama.