New U.S. research has found that using marijuana at an increasing rate over time during adolescence may be associated with an increased risk of depression in later life and a lower number of educational accomplishments.

Carried out by researchers from the Department of Psychiatry at the University of Pittsburgh School of Medicine and Pitt Department of Psychology, the team analyzed 158 boys and young men who were part of The Pitt Mother & Child Project (PMCP).

The project is a long-running longitudinal study of males in the Pittsburgh area who are at high risk for antisocial behavior and other conditions based on low income, family size and child gender.

"We know that cannabis use in adolescence is associated with outcomes like lower educational level, and difficulties with mood and depression, but through this long-term study, we've been able to provide a much deeper insight into this relationship, showing that certain characteristics of use may be more important than others," said lead author Erika Forbes.

The team looked at how often the participants used cannabis between the ages 14 to 19 to see if there was a relationship between using cannabis during teenage years and psychosocial outcomes.

Participants were also asked to report at age 20 their cannabis use and habits for every year since they started using the drug, and at age 20 and 22 completed questionnaires measuring depression and educational attainment.

Each participant's brain was also scanned using fMRI to assess functional connectivity in the brain's reward circuit.

The researchers were surprised to find that boys who started using cannabis occasionally around 15 or 16 years old but then increased their use dramatically by 19 years old had the greatest dysfunction in brain reward circuitry, the highest rates of depression and the lowest educational achievements.

"We expected to see that the young men who had a high, consistent level of marijuana use would have differences in brain function. However, it turned out that those who had an increasing pattern of use over their teens had the biggest differences," explained Forbes.

"Though the results do not show a direct causal link, it's important to note that even though most people think marijuana isn't harmful, it may have severe consequences for some people's functioning, education and mood," continued Forbes.

"While that may seem unimportant at age 20, the level of education you receive will likely have a huge effect on your quality of life and socioeconomic status later in adulthood."

The findings can be found published online in the journal Addiction.