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.
For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand.
They assessed them as children - before any of them had started using cannabis - and then re-interviewed them repeatedly, up to the age of 38.
Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education, they found that those who persistently used cannabis - smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s - suffered a decline in their IQ.
Tuesday, August 28, 2012
No big surprise
BBC News - Young cannabis smokers run risk of lower IQ, report claims