Yes, despite what potheads claim. Doctors in Greece compared the mental abilities of 20 people who had smoked dope four times a week for 15 years with 20 who had used it for less than seven years, and 24 never-smokers. They were given 15 words to learn, and asked to repeat them later. The average score for the long-term smokers was 7; for the shorter-term smokers, 9; for the never-users, 12. It is the latest in many studies showing repeated ‘soft’ drug abuse damages the brain. This isn’t surprising because marijuana’s active ingredient, tetrahydro cannabinol (THC), is highly fat-soluble. As our brain is the organ with the highest concentration of fat, THC makes a beeline for it and stays there for

Source: The Guardian Saturday September 30, 2006