Rand Corporation, Drug Policy Research Center 2004. The study used survey data from 5,833 California and Oregon middle-school students. The participants completed the surveys six times over a 10-year period between ages 13 and 23. About 44% also responded to survey questions at age 29 with data from age-matched abstainers. 
Findings:

• Report suggests that people who abstain from smoking marijuana appear to have a better quality of life and higher levels of educational achievement than marijuana users

• Early use of marijuana was associated with lowered income and reduced health later in life

• The abstainers had an overall higher level of educational attainment, better health, greater life satisfaction and a lower rate of other drug use

• Those reporting a relatively high level of use at age 13 fared significantly worse than all other groups on over-all health and yearly earnings. The results show that people underestimate the harm marijuana can cause.