those using marijuana scored approximately one third lower than non-users

New research shows that people who start using marijuana at a young age and those who use the greatest amount of marijuana may be the most cognitively impaired.  The research was presented at Neuroscience 2010, the annual meeting of the Society for Neuroscience, held in San Diego. Marijuana users show deficits in the ability to switch behavioral responses according to the context of a situation, also known as cognitive flexibility. The new study, directed by Staci Gruber, PhD, at McLean Hospital and Harvard Medical School, compared people’s performance on the Wisconsin Card Sorting Task, a test of cognitive flexibility. During the...

Regular cannabis users who start using the drug before the age of 15 perform worse on brain tests than those who start later, according to new research published in the June issue of the British Journal of Psychiatry. Researchers from the Universidade Federal de Sao Paulo say their study suggests that early cannabis use may have more harmful effects on people’s cognitive functioning. The researchers asked 104 chronic cannabis users to take part in a series of neuropsychological tasks. These tested their executive functioning, attention, perseverance, ability to form abstract concepts, visual and motor skills, and mental...