This widely publicised study found that younger teenagers who regularly use marijuana could face a significant drop in their IQ. Over 1,000 New Zealand children were tested at 13 and then 38 years of age and regularly interviewed. Those classed as dependent suffered an average 8 point drop in IQ, even if they later quit. Those who became dependent after 18 years did not suffer the same decline. Although the findings are not definitive they highlight the importance of this field of research.
Teen pot use linked to later declines in IQ - Associated Press