FRIDAY, Dec. 14, 2012 (HealthDay News) -- Teens who are heavily into drinking and smoking pot may show signs of breakdown in the brain's wiring system, a small study suggests.
Using brain scans of 92 teenagers, researchers found that kids who regularly drank and used marijuana showed negative changes in the brain's "white matter" over 18 months.
The brain has two broad types of tissue, known as gray matter and white matter. The gray matter can be seen as the brain's information-processing centers, while the white matter is like the wiring connecting those centers.
It's not clear what the current findings could mean for teens' everyday brain function. And it's not even certain that it's the substance abuse causing the white matter changes.
But researchers say the results offer a cautionary message about heavy drinking and pot smoking.
"White matter is the information highway. It allows the brain to communicate quickly and efficiently," said the study's lead researcher, Joanna Jacobus, a postdoctoral fellow at the University of California, San Diego.
If white matter is "less healthy," she explained, there could be subtle effects on a person's memory, attention and mental processing speed.
Dr. Duncan Clark, a researcher at the University of Pittsburgh School of Medicine who studies teen substance abuse, also weighed in on the study.
"We are concerned that alcohol, marijuana or other substance use may cause delays or deficits in teen brain development," Clark said. "This study adds to those concerns."
Other studies have found signs of white-matter "disorganization" in kids who drink or smoke pot. What's different here is that the researchers followed kids over time to see whether substance abuse itself was linked to brain changes, said Clark, who was not involved in the study.
The findings, reported online Dec. 14 in the journal Alcoholism: Clinical and Experimental Research, are based on 41 teenagers with a history of habitual drinking and pot smoking, and 51 teens who reported little, if any, alcohol or drug use.
At the start of the study and again 18 months later, all of the teens underwent a type of MRI scan that maps the diffusion of water molecules in the brain. The technique can pick up subtle changes in the structural integrity of the brain's white matter.
As a group, Jacobus and her colleagues found, kids who drank and smoked pot showed negative white-matter changes. In particular, the more they drank over the study period, the worse their white-matter integrity.
Over 18 months, teens in the substance-abuse group drank on more than 400 occasions, on average, and smoked pot more than 650 times. Heavier marijuana use, however, did not correlate with the negative brain changes.
"This study suggests that alcohol use has more effect than marijuana use," Clark said.
He added, though, that more research is needed on that question -- including whether drinking and pot smoking together could have some combined effects. "I believe it would be premature to make conclusions about marijuana's effects," Clark said.
The researchers had no information on what the teens' white matter was like before they started drinking and smoking pot. So they can't say for sure whether the drugs are the cause of the brain changes they saw.
It's always possible there are other explanations, Jacobus said.
"Genetics and home environment can influence healthy brain development in many ways," she said. "It is possible that white matter and other structural brain differences between these youth predisposed certain individuals to use more heavily compared to others."
The differences in white matter between substance abusers and nonusers might translate into only subtle effects in real life -- in areas such as kids' school performance, Jacobus said.
But, she said, "it's important to point out that with regular, repeated, heavy use throughout adolescence and young adulthood, these small effects may become more noticeable and consequential."
Learn more about the effects of substance abuse from the U.S. National Institute on Drug Abuse.