Long-held fears that the use of marijuana will lead to harder drugs are overblown, according to new research from the University of New Hampshire.

Marijuana protest

A young woman smokes a joint outside the Vancouver Art Gallery. New research suggests use of marijuana as a teen is not a major factor in using hard drugs later in life. (Jonathan Hayward/Canadian Press)

The research, in the September issue of the Journal of Health and Social Behavior, found that other factors, such as whether or not a person has a job, or is facing severe stress, are far more predictive of future hard drug use than whether they smoked pot as a teenager.

"Employment in young adulthood can protect people by closing the marijuana gateway, so over-criminalizing youth marijuana use might create more serious problems if it interferes with later employment opportunities," said co-author Karen Van Gundy.

The strongest factor influencing the use of illicit drugs is an individual's race or ethnicity, according to the study. Non-Hispanic whites are most likely to use harder drugs such as heroin or cocaine, followed by Hispanics and then by African Americans.

'We urge U.S. drug control policymakers to consider stress and life-course approaches in their pursuit of solutions to the drug problems' —Karen Van Gundy, Cesar Rebellon, University of New Hampshire

Young adults who didn't complete high school or go to college were most likely to have used marijuana as teens and other illicit drugs in early adulthood. Those who were unemployed after high school were also more likely to use other drugs.

"In light of these findings, we urge U.S. drug control policymakers to consider stress and life-course approaches in their pursuit of solutions to the drug problems," write the study's authors, Van Gundy and Cesar Rebellon, both associate professors of sociology at UNH.

The researchers also found that any gateway effect that does exist with marijuana disappears once young adults reach 21.

"While marijuana use may serve as a gateway to other illicit drug use in adolescence, our results indicate that the effect may be short-lived," note the authors.

"Interestingly, age emerges as a protective status above and beyond the other life statuses and conditions considered here."

The researchers followed 1,286 young adults who attended Miami-Dade public schools in the 1990s. Twenty-six per cent were African American, 44 per cent Hispanic and 30 per cent non-Hispanic white.