Teenagers who smoke cannabis could permanantly reduce their IQ a new study has shown.

The study included 1,000 people in New zealand and their results showed that those who started smoking cannabis under the age of 18 suffered a decrease in IQ.

An expert has said that this study could be the answer to why students who take the drug underachieve.

The study had followed people whom were in their teens up until they turned 38 with interviews.

They found that people who smoked more than 4 times a week had a drop in IQ and when tried to cut down and completely stop their was no re storage of their IQ.