From the July 14, 2003 issue of Business Week:

Scientists know that many illegal drugs, as well as alcohol, can cause irreversible brain damage. But according to a recent study, marijuana poses no such threat. Researchers from the University of California at San Diego compared the mental performance of 704 long-term pot smokers, who were tested while not high, with 484 nonusers. They found that even heavy marijuana use did no permanent harm to a variety of key functions including language, reaction time, motor skills, or perceptual and reasoning abilities. The only exception: a small decrease in the pot smoker's ability to learn new information. The findings, published in July's Journal of the International Neuropsychological Society, suggest that short-term use of cannibas for medical purposes is unlikely to cause harmful side effects.