Longitudinal studies track people over long periods of time. The 9/14 edition of The Lancet Psychiatry includes an integration of three longitudinal studies regarding marijuana use among teens in Australia and New Zealand. Between 2,537 and 3,765 teens (depending on the outcome variable), who used varying degrees of marijuana (from none to daily) were tracked to age 30. Those teens who were daily users of marijuana before age 17 had much lower educational outcomes and had significantly higher rates of later substance abuse problems and suicide attempts. Quoting directly from the article: “…individuals who were daily users before age 17 years had odds of high-school completion and degree attainment that were 63% and 62% lower, respectively, than those who had never used cannabis; furthermore, daily users had odds of later cannabis dependence that were 18 times higher, odds of use of other illicit drugs that were eight times higher, and odds of suicide attempt that were seven times higher.” However, teens who smoked less were also negatively affected; the dose of the poor outcome was directly related to the dose of the marijuana use. The authors conclude as follows: “Study findings suggest that adolescent cannabis use is linked to difficulties in successfully completing the tasks that mark the transition to adulthood. Prevention or delay of cannabis use in adolescence is likely to have broad health and social benefits.”
The Lancet study brings to mind a report released by the National Institutes of 
Health in December of 2013. This report, which summarized the results of a large national survey, indicated that 6.5% of high school students reported smoking marijuana daily. Moreover, nearly one out of four seniors reported having smoked it in the preceding month, with only 39.5% of them viewing regular marijuana use as being harmful. Bolstered by movements in the U.S. to legalize marijuana, many teens may argue that marijuana use is harmless. The important research reported in the Lancet would suggest otherwise. Moreover:
• No state that has legalized marijuana use for adults has done so for minors. Teens who smoke pot risk facing legal consequences in every state. For instance, in Pennsylvania, teens caught with marijuana are at risk to loose their driver’s license, among other consequences.
• What’s legal ≠ what’s healthy.
• Human brains continue to develop into early to mid twenties. And, the part of the brain that develops last is responsible for the most sophisticated and higher order brain functions. I know of no reputable scientist or clinician who would argue that it is advisable to introduce any psychoactive agent into a developing brain unless there is a compelling and well thought out need to treat a well-diagnosed condition. Teen life is challenging and complex enough without adding such a wildcard.
• There is evidence that people with genetic predispositions to certain disorders can have them activated by significant marijuana abuse (e.g., schizophrenia).
I think we parents need to insist that marijuana use among our teens is NOT okay. Please see my blog articles on monitoring and discipline strategies for support. However, if your teen is abusing marijuana, or any substance, and you are finding you cannot change this, please seek out the services of a qualified mental health professional. For a referral click here.
Dr. David Palmiter's Blog for Hectic Parents 