Marijuana is the most commonly used illicit drug in the U.S. — for the population overall and for youths in particular.

Based on 2013-2014 data, 7.22 percent of adolescents aged 12 to 17 years across the nation had used marijuana in the past month. Of adults 26 or older who used marijuana before age 15, 62 percent then went on to use cocaine at some point in their lives, and 9 percent eventually used heroin at least once.

Researchers have discovered that regular use of cannabis, or marijuana, during the teen years is associated with a greater risk using illicit drugs as adults.

One study followed cannabis use among 5,315 teens (ages 13 to 18 years) five times, one year apart. The teens reported use as none, occasional (less than once a week) or frequent (once a week or more). By age 21, 462 reported addtional illicit drug use: cocaine (38 percent), amphetamines (60 percent), inhalants (30 percent), sedatives (16 percent), hallucinogens (23 percent) and opioids (6 percent).

Monitoring trends in adolescent marijuana use remains important because of associated health risks. A new study suggests that cannabis may be a trigger for schizophrenia, a complex mental disorder that usually presents symptoms between ages 16 and 30 years.

Schizophrenia occurs in 1.1 percent of the population, or 3.2 million Americans over the age of 18. Researchers believe that a genetic disposition combined with environmental factors may contribute, or trigger, the brain disorder.

To that point, the study conducted by Dr. Ran Barzilay, a child and adolescent psychiatrist, and led by Prof. Dani Offen, both of Tel Aviv University's Sackler School of Medicine, suggests that smoking marijuana or using cannabis in other ways during adolescence may be a trigger for those who are already susceptible to schizophrenia.

The researchers used a unique mouse model based on host (DISC1) X environment interaction to study the pathobiological basis through which exposure to tetrahydrocannabinol (THC), the psychoactive compound in cannabis, elicits psychiatric manifestations. THC interferes with the endogenous endocannabinoid system, which is critically involved in both pre- and post-natal neurodevelopment.

The researchers used four categories of mice: genetically susceptible and exposed to cannabis, genetically susceptible and not exposed to cannabis, genetically intact and exposed to cannabis, and genetically intact and not exposed to cannabis.

The behavioral tests and neurological biochemical analyses showed that only the genetically susceptible mice developed behavioral and biochemical brain pathologies mimicking the first episode of schizophrenia. In the nonsusceptible group of mice, a protective mechanism was observed, involving the upregulation of BDNF, a neurotrophic factor in the hippocampus.

This finding is important for the development of future compounds that could reduce the negative effect of cannabis on brain development.

Offen warns that until such compounds are developed, young people at risk for psychiatric disorders, such as schizophrenia, should rethink using cannabis during adolescence.