Since 1996, 18 states plus the District of Columbia have passed laws legalizing marijuana use for a variety of medical conditions. As of today, six states — Illinois, Minnesota, New Hampshire, New York, Ohio, and Pennsylvania — have pending legislation to approve medical marijuana. Last November, voters in Colorado and Washington became the first to sanction measures that legalize cannabis for non-medical purposes.

Clearly, voters are indicating a desire to relax marijuana laws, yet the use of this drug continues to be an offense under federal law. Classified as a Schedule I drug, marijuana is deemed to have a high potential for abuse; no currently accepted medical use in treatment in the U.S.; a lack of accepted safety for use of the drug under medical supervision. Meanwhile, marijuana is the second most popular recreational drug in the U.S., with alcohol remains king among teens and emerging adults; almost a third of 12th graders and 40 percent of college students report recent binge drinking.

How do the two substances stack up?

Alcohol Versus Cannabis: A Comparison Of Effects On The Brain

A recent study conducted by researchers at the University of California found that cannabis use among those ages 16-20 over an 18 month period had a less negative effect on a teenager's brain tissue than drinking alcohol.

For the study, researchers scanned the brains of 92 individuals ages 16-20, before and after an 18 month period. During the eighteen months, half of the teens used cannabis and alcohol in varying amounts, while the other half abstained or kept consumption minimal. Among those who consumed five or more drinks at least twice a week, researchers discovered reduced brain tissue health. Specifically, those consumption patterns compromised the integrity of white brain tissue in specific tracts, which could lead to declines in memory as well as decision-making ability. However, among those who used marijuana, there were no findings of ill effects on white tissue during the period of scanning.

Although it may be interesting to note that smoking pot has less adverse effects on brain tissue, this study finding alone should not (excuse the pun) 'cloud' the issue of public health. Another study, one that involved a review of data from Australia, indicates some negative mental health issues linked to marijuana use.

Cannabis, Major Depressive Disorder, And Suicidal Thoughts

Dr. Michael Lynskey and his colleagues at the Washington University School of Medicine in St. Louis gathered data from four groups of same-sex twin pairs (508 identical, 493 fraternal; 518 female, 483 male) enrolled in the Australian Twin Registry. The researchers discovered that individuals who are dependent on cannabis have a higher risk than non-dependent individuals of experiencing major depressive disorder and suicidal thoughts and behaviors. They also discovered that men and women who smoked marijuana before age 17 are 3.5 times as likely to attempt suicide as those who started later.

Although the causal relationship between marijuana use and mental illness is unclear, the researchers suggest their findings demonstrate the importance of considering mental health issues in the treatment and prevention of marijuana abuse. Significantly, among the 156 pairs discordant for diagnosis of major depressive disorder (MDD) before age 17 — meaning, one twin was diagnosed as depressed, the other was not — fraternal but not identical twins with early diagnosis of MDD were 9.5 times as likely to develop marijuana dependence. Among the 257 pairs discordant for having suicidal thoughts before age 17, fraternal but not identical twins with early suicidal thoughts were 5.5 times as likely as their twins to become dependent on marijuana.

The fact that two of the relationships were observed in fraternal but not identical twins suggests that the experiences related in each — marijuana dependence and MDD, and marijuana dependence and suicidal thoughts — may share a common underlying genetic basis, notes Lynskey.

This theme of marijuana's impact on mental health, along with other adverse outcomes, arises in other studies of cannabis use as well.

Other Effects Of Marijuana On The Body

"The most probable adverse effects include a dependence syndrome, increased risk of motor vehicle crashes, impaired respiratory function, cardiovascular disease, and adverse effects of regular use on adolescent psychosocial development and mental health," writes Professor Louisa Degenhardt, who conducted research on illicit drug use for the National Drug and Alcohol Research Centre (NDARC) of Australia and other projects. Her review of marijuana concludes that acute adverse effects of cannabis use include anxiety and panic in naive users. Meanwhile, she also discovered that use during pregnancy may reduce birthweight, but does not seem to cause birth defects. "Whether cannabis contributes to behavioural disorders in the offspring of women who smoked cannabis during pregnancy is uncertain," she writes.

Chronic cannabis use can produce a dependence syndrome in as many as one in 10 users. Regular users have a higher risk of chronic bronchitis and impaired respiratory function, and psychotic symptoms and disorders. Many exhibiting mental disorders have a history of psychotic symptoms or a family history of these disorders. Although she finds that regular cannabis use in adolescence indicates a tendency to use other illicit drugs, she states the 'most probable adverse psychosocial effect' in adolescents who become regular users "is impaired educational attainment."

For cognitive performance, the size and reversibility of any impairment remain unclear. That said, Degenhardt notes that a recent Australian study estimated that cannabis use caused zero to two percent of total disease burden in Australia, which is a country with one of the highest reported rates of cannabis use.

Because recent legislation will greatly impact public health, the effects of cannabis use, whether for medical or recreational purposes, needs to be better understood. Further research into the effects of marijuana, whether it remains illicit or not, on the developing brains of adolescents is especially relevant to questions of public health.

Sources: Bava S, Jacobus J, Thayer RE, Tapert SF. Longitudinal Changes in White Matter Integrity Among Adolescent Substance Users. Alcoholism: Clinical and Experimental Research. 2013.

Degenhard L. Adverse health effects of non-medical cannabis use. The Lancet. 2009.

Lynskey MT, Glowinski AL, Todorov AT, Bucholz KK, Madden PAF, Nelson EC, et al. Major depressive disorder, suicidal ideation, and suicide attempt in twins discordant for cannabis dependence and earlyonset cannabis use. Archives of General Psychiatry. 2004.