A team of international researchers say persistent marijuana use by young teenagers can cause a drop in IQ later on in life.
Researchers say teenagers who become hooked on marijuana before they reach 18 may be causing lasting damage to their intelligence, memory and attention.
Researchers from New Zealand, Britain and the U.S. found that dependent use of marijuana as a teen may have a so-called neurotoxic effect.
However, they also found heavy pot use after age 18 appears to be less damaging to the brain.
The team studied the records of more than a thousand New Zealanders from 1972 onwards.