SALT LAKE CITY — Marijuana is harmful to teen brain development and may lead to long-term IQ decline, new research shows.

The researchers tracked the habits of 1,037 people, from birth to age 38, examining how often they smoked marijuana. The participants who heavily used marijuana from high school through their late 30s scored 8 points lower on an IQ test than their score from when they were 13 years old. Their abstinent peers showed no change, the New York Times reported.

"Pot smoking is relatively common in American teens," the Associated Press reported. "The government reported in June that 23 percent of high school students said they'd recently smoked marijuana, making it more popular than cigarettes."

The research did not go far enough to determine the minimum dosage of marijuana associated with such medical effects, CNN reported. "It's not clear from the findings whether less-frequent users might experience similar declines in IQ and mental function."

The research, which appears in Proceedings of the National Academy of Sciences, indicated that heavy pot smokers showed memory loss, concentration problems and general cognitive deficits. For those who had begun smoking marijuana as teens, however, these effects were worse and longer-lasting, CNN observed.

The study "doesn't prove that marijuana use directly impairs intelligence. It does, however, provide some of the strongest evidence to date of a cause-and-effect relationship," CNN reported.

"Adolescent-onset cannabis users showed significant IQ declines, and more persistent use was associated with greater declines," lead author Madeline H. Meier, a postdoctoral researcher at Duke University, told the Associated Press. "We know that there are developmental changes occurring in the teen years and up through the early 20s, and the brain may be especially vulnerable during this time."

Rachel Lowry is a reporter intern for the Deseret News. She has lived in London and is an English graduate from Brigham Young University. Contact her at rachel.lowry@gmail.com or visit www.rachellowry.blogspot.com.