Using marijuana during adolescence does not appear to increase the risk for later physical or mental health issues such as depression, psychotic symptoms or asthma, according to a new study, which flies in the face of some prior research.

"What we found was a little surprising," lead researcher Jordan Bechtold, PhD, a psychology research fellow at the University of Pittsburgh Medical Center, in Pennsylvania, said in a news release. "There were no differences in any of the mental or physical health outcomes that we measured regardless of the amount or frequency of marijuana used during adolescence."

The study was published online August 3 in Psychology of Addictive Behaviors, a journal of the American Psychological Association.

No Significant Differences

As part of the Pittsburgh Youth Study, the researchers tracked 408 boys from adolescence into their mid-30s. The study sample was 54% black, 42% white, and 4% other races or ethnicities.

They were divided into four groups on the basis of their reported marijuana use: low use or nonuse (46%); early long-term use (22%); those who only smoked marijuana during adolescence (11%); and those who began using marijuana later in their teen years and continued using the drug (21%).

After controlling for multiple potential confounding variables, such as use of alcohol, tobacco, and hard drugs and socioeconomic status, long-term marijuana users were not more likely than late increasing users, adolescence-limited users, or low/nonusers to suffer several physical or mental health problems in their mid-30s.

"In fact, there were no significant differences between marijuana trajectory groups in terms of adult health outcomes, even when models were run without controlling for potential confounds," the researchers note in their article.

"This is particularly striking given that men in the early onset chronic group were using marijuana (on average) once per week by late adolescence and continued using marijuana approximately 3 -4 times a week from age 20 to 26 years," they write. There were no differences in the findings based on race or ethnicity.

The mental health outcomes included anxiety and mood and psychotic disorders. The physical health outcomes included asthma, allergies, headaches, high blood pressure, limitations in physical activities, physical injuries, and concussions.

Interpret With Caution

"We wanted to help inform the debate about legalization of marijuana, but it's a very complicated issue, and one study should not be taken in isolation," Dr Bechtold said.

"This does not discredit the work of others," the authors note. "It could be the case that cumulative tetrahydrocannabinol exposure, age of initiation of use, or use at one particular age is more predictive of negative health outcomes than the overall pattern of use between adolescence and adulthood."

Marijuana policy makers and stakeholders "need to consider the results of any single study in the context of the larger body of work on the potential adverse consequences of early onset chronic marijuana use," they conclude.

April Thames, PhD, of the Department of Psychiatry and Biobehavioral Sciences, University of California, Los Angeles, who was not involved in the study, told Medscape Medical News, "This is an interesting and well-designed study that addresses many of the limitations from past studies. The authors did an excellent job of pointing out limitations of their study (eg, use of self- report measures of use, lack of collecting measures on quality and potency of marijuana).

"As a neuropsychologist, one important question that comes to mind is, How does heavy use during adolescence influence cognitive functioning over time? There is recent evidence to suggest that chronic, heavy use during adolescence is associated with lower IQ scores," she added.

Dr Thames also wonders whether any potential physical health ramifications would present in older adulthood, beyond the mid-30s. "We know that normal aging is associated with declines in immune functioning, so it is still unclear whether or not chronic/heavy marijuana use interacts with or accelerates the aging process," she said.

"These are questions that remain, although this is an excellent start to addressing the various health and mental health concerns surrounding the use of marijuana among teens," said Dr Thames.

The study was supported by the National Institute on Drug Abuse, the National Institute of Mental Health, the Pew Charitable Trusts, the Office of Juvenile Justice and Delinquency Prevention, and the Pennsylvania Department of Health. The authors have disclosed no relevant financial relationships.

Psychol Addict Behav. Published online August 3, 2015. Full text