From the NIDA website "Research clearly demonstrates that merijuana has the potential to cause problems in daily life or make one persons existing problems worse. In one study, marijuana abusers reported that the drug impaired several important measures of life achievement including physical and mental health, cognitive abilities, social life, and career status. Several studies associate workers' marijuana smoking with increased absences, tardiness, accidents, workers' compensation claims and job turnover."
Check out the website yourself
Studies:
Gruber, AJ, Pope HG...Attributes of long-term heavy cannibis users: A case control study. Psychological Med 33(8):1415-1422, 2003.
And I'm done