Daily Mail:  “Marijuana use does NOT affect teenager’s IQ, says new study (but it does make you dumber as an adult).  Using cannabis between 12 and 18 does not make an adolescent’s IQ decline
Children with low IQs are more likely to take the drug as teenagers. Cannabis does not have the suspected effects on brains and education. Researchers from Arizona State University analyzed 1,989 twins born in 1994-5. Past studies revealed cannabis use affects teenager’s educational achievements.”