When a teenager smokes cannabis are they permanently damaging their brain and dulling their intellect for a lifetime?

The dangers of smoking cannabis, and the potential health benefits, have been a source of controversy for many years. The latest study on the drug suggested heavy and prolonged cannabis smoking as a teenager resulted in a permanently lower IQ.

But how could cannabis have this effect on the brain and why might teenagers be particularly susceptible?

via BBC News – How could cannabis alter the teenage brain?.