Harrison Pope, a Harvard professor of psychiatry, and his colleagues at McLean Hospital, a Harvard-affiliated psychiatric facility in Belmont, Mass., investigated the long-term cognitive effects of smoking marijuana. They recruited 180 people, who ranged from light to heavy users. Heavy use was defined as smoking pot at least 5,000 times. All the research subjects took batteries of intelligence, attention, learning, and memory tests on days zero, one, seven, and 28 after quitting the drug. On days zero, one and seven, current heavy smokers scored significantly lower than the other groups on memory tests. “By day 28, however, there were no significant differences among the groups on any of 10 different tests, and no significant association between cumulative lifetime marijuana use and test scores,” Pope says. The researchers concluded that heavy marijuana use produces no irreversible mental deficits. But they cannot say for sure why pot smokers remain impaired for days or weeks after giving up the drug.