http://www.nida.nih.gov/Infofacts/marijuana.html
Effects of Heavy Marijuana Use on Learning and Social Behavior
Depression(19), anxiety(20), and personality disturbances(21) have been associated with marijuana use. Research clearly demonstrates that marijuana has potential to cause problems in daily life or make a person’s existing problems worse. Because marijuana compromises the ability to learn and remember information, the more a person uses marijuana the more he or she is likely to fall behind in accumulating intellectual, job, or social skills. Moreover, research has shown that marijuana’s adverse impact on memory and learning can last for days or weeks after the acute effects of the drug wear off(22, 23).
Students who smoke marijuana get lower grades and are less likely to graduate from high school, compared with their non-smoking peers(24, 25, 26, 27). A study of 129 college students found that, for heavy users of marijuana (those who smoked the drug at least 27 of the preceding 30 days), critical skills related to attention, memory, and learning were significantly impaired even after they had not used the drug for at least 24 hours(28). The heavy marijuana users in the study had more trouble sustaining and shifting their attention and in registering, organizing, and using information than did the study participants who had used marijuana no more than 3 of the previous 30 days. As a result, someone who smokes marijuana every day may be functioning at a reduced intellectual level all of the time.
More recently, the same researchers showed that the ability of a group of long-term heavy marijuana users to recall words from a list remained impaired for a week after quitting, but returned to normal within 4 weeks(29). Thus, it is possible that some cognitive abilities may be restored in individuals who quit smoking marijuana, even after long-term heavy use.
Workers who smoke marijuana are more likely than their coworkers to have problems on the job. Several studies associate workers’ marijuana smoking with increased absences, tardiness, accidents, workers’ compensation claims, and job turnover. A study of municipal workers found that those who used marijuana on or off the job reported more “withdrawal behaviorsâ€ÂÂâ€â€Âsuch as leaving work without permission, daydreaming, spending work time on personal matters, and shirking tasksâ€â€Âthat adversely affect productivity and morale(30). In another study, marijuana users reported that use of the drug impaired several important measures of life achievement including cognitive abilities, career status, social life, and physical and mental health(31).