While smoking marijuana is now legal in a couple of states, researchers say it may have negative health effects on teenagers.
Researchers at Northwestern University studied nearly 100 people who smoked pot every day for at least two years when they were teenagers.
Scans revealed their brains had abnormalities that impaired their memory, even after they stopped using the drug.
It appears the memory structures in their brains collapsed and shrank.
Also, the younger the teenagers were when they smoked, the greater the damage that was done.