POT MYTH BLOWN - IQ is not affected by using POT. Alcohol does lower IQ though.

http://www.alternet.org/drugs/science-busts-one-most-common-myths-about-smoking-pot

Moderate cannabis consumption by young people is not positively associated with changes in intelligence quotient (IQ), according to  data presented this week at the European College of Neuropsychopharmacology annual congress in Berlin, Germany.
Investigators at the University College of London analyzed data from 2,612 subjects who had their IQ tested at the age of eight and again at age 15. They reported  no relationship between cannabis use and lower IQ at age 15 when confounding factors such as subjects’ history of alcohol use and cigarette use were taken into account.