Two recent studies, one in Great Britain and this one from the University of Southern California, contradict the findings of a rigorous 25-year-long study done with a birth cohort in Dunedin, New Zealand a few years ago. That study found that persistent marijuana use that continued into adulthood resulted in an 8-point drop in IQ. The two new studies find the opposite.

The UCLA study looked at 789 pairs of adolescent twins from two ongoing studies—one in Los Angeles and one in Minnesota—who enrolled between ages 9 and 11. Over 10 years, five IQ tests were administered along with confidential surveys of marijuana use. Marijuana-using twins lost 4 IQ points, but so did their non-using twins, leading researchers to conclude that something other than marijuana was lowering IQ.

The other study compared teens who reported daily marijuana use for six months or longer with teens who used the drug less than 30 times and found no difference in IQ.
 
But critics say both studies are flawed in that they did not measure heavy marijuana use over a long 25-year period like the Dunedin study did.
 
Dr. Madeline Meier, lead researcher of the Dunedin study, writes, “Our 2012 study (Meier et al. PNAS 2012) reported cognitive decline among individuals with a far more serious and far more long-term level of cannabis use. That is, we found cognitive decline in individuals followed up to age 38 who started cannabis use as a teen and who thereafter remained dependent on cannabis for many years as an adult. This new study is different; the two papers report about completely different doses of cannabis, and about participants 2 decades apart in age.  The new study reports cognitive test scores for individuals followed up to only age 17-20, fewer than half of whom had used cannabis more than 30 times, and only a fifth of whom used cannabis daily for > 6 months. This new study and our prior study agree and both report the same finding: no cognitive decline in short-term low-level cannabis users. The message from both studies is that short-term, low-level cannabis use is probably safer than very long-term heavy cannabis use. The big problem remains that for some teens, short-term low-level teenaged cannabis use leads onward to long-term dependence on cannabis when they become adults. That is what is cause for concern.”
 
Read Science story here. Read Dr. Meier’s rebuttal here.

Source: Email from Monte Stiles, National Families in Action January 2016