http://www.bbc.co.uk/news/health-19372456
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 around 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.
A UK expert said the research might explain why people who use the drug often seem to under-achieve.