Cannabis, IQ and the law

Heavy marijuana use is associated with cognitive decline in about 5% of teens, according to a new study, which suggests that the heaviest users could lose 8 IQ points.

In the report, published in the Proceedings of the National Academy of Sciences, research conducted in New Zealand showed that teens who started smoking marijuana before age 18 and were diagnosed as being addicted to cannabis by age 38 experienced an IQ drop in early adulthood. But users who began smoking after age 18 - even if they used heavily - did not show a significant decline.

“The effect of cannabis on IQ is really confined to adolescent users,” says lead author Madeline Meier, a postdoctoral researcher at Duke University, “Our hypothesis is that we see this IQ decline in adolescence because the adolescent brain is still developing and if you introduce cannabis, it might interrupt these critical developmental processes.”

The authors followed 1037 children born in the town of Dunedin, New Zealand in 1972 and 1973, virtually every child. They defined adolescent use as at least weekly use before turning 18. In looking at the relationship between marijuana use and IQ, they controlled for factors like years of education, schizophrenia and use of alcohol or other drugs that might also have an effect on IQ. While education weakened the relationship, it still did not eliminate it.

Researchers also had family members and friends of the participants confidentially rate them on attention and memory skills and those who had lost IQ points showed problems in these areas. Meier notes that an 8 point decline in IQ for someone with average intelligence (an IQ score of 100; the 50th percentile) would move that person down to the 29th percentile. “It’s fairly substantial but it does depend on where you start out,” she says.

Legality of cannabis by U.S. jurisdiction The use, possession, sale, cultivation, and transportation of cannabis (marijuana) in the United States under federal law is illegal but the federal government has announced that if a state wants to pass a law to decriminalize cannabis for recreational or medical use they can do so, but they need to have a regulation system in place for cannabis. Cannabis is listed as a Schedule I substance under the Controlled Substances Act of 1970, the highest classification under the legislation. This means that the substance has been claimed by the U.S. federal government to have both high abuse potential and no established medical use.

However, individual state laws do not always conform to the federal standard. State-level proposals for the rescheduling of cannabis have met with mixed success. Currently, the use of both recreational and medicinal marijuana has been entirely legalized in the states of Colorado, Washington, Alaska and Oregon. The cities of Portland and South Portland in Maine fully legalized marijuana for both medical and recreational use. The District of Columbia has fully legalized recreational and medical marijuana, but recreational commercial sale is currently blocked by Congress. Eleven states have both medical marijuana and decriminalization laws. Nine states and Guam have only legalized medical marijuana. Three states and the U.S. Virgin Islands have only decriminalized possession laws. The remaining twenty-two states and three inhabited territories state that marijuana is illegal, a felony, or a misdemeanor.