Marijuana is the most widely used illegal drug in America and the most commonly used drug among young adults aged 18-25.

Now a new study finds the earlier a user starts, and the longer he smokes, the more damage he may do to his brain.

Researchers at Duke University analyzed data from more than one-thousand New Zealand natives born in 1972 and 1973. They were all followed until they turned 38.

The researchers found that those who started smoking marijuana early in life and kept using the drug throughout the years showed a greater decline in IQ compared with other participants.

Probing deeper, the data showed that persistent pot use caused problems with cognitive functioning,trouble wiyh attention and memory.

Teenage marijuana users tended to become more persistent users and experienced a greater decline in IQ than adult onset users. This led the researchers to speculate that cannabis use in adolescence, when the brain is developing, could have irreversible neurotoxic effects.

The complete study can be found in the Proceedings of the National Academy of Sciences.

I'm Dr. Cindy Haines, with health information for your entire family.