Young cannabis smokers run risk of lower IQ, report claims
28 August 2012 Last updated at 19:08 BST
Young people who smoke cannabis run the risk of a significant and irreversible reduction in their IQ, research suggests.
The findings come from a study of about 1,000 people in New Zealand.
An international team found those who started using cannabis below the age of 18 - while their brains were still developing - suffered a drop in IQ.
Mark Easton reports.