Dr. Gilman, an instructor in psychology at the Massachusetts General Hospital - Harvard Center for Addictions Medicine, and fellow researchers at Harvard and Northwestern University found structural changes in  the brains of participants who smoked marijuana. There were even differences in subjects that smoked only once or twice a week.

"The fact that we can see these structural changes in the brain could indicate that the effects of THC are longer lasting than we thought," said Dr. Gilman. Last year at Northwestern, researchers also saw changes in the nucleus accumbens among adults in their early 20s who had smoked daily for three years but had stopped for at least two years. They had impaired working memories as well.

Read the article in the New York Times.

E-cigarettes may increase the risk of addiction to cocaine and other drugs, nicotine researchers suggest in this week's New England Journal of Medicine. While e-cigarettes eliminate some of the health risks associated with regular cigarettes, they are pure nicotine-delivery devices, with the same effects on the brain as cigarettes, according to the wife-husband research team Denise and Eric Kandel. They say e-cigarettes can serve as a "gateway drug," making people more likely to use and become addicted to drugs, Time reports.

The Kandels note that epidemiologic studies have shown that nicotine use is a gateway to the use of marijuana and cocaine in human populations. In earlier research in mice, the Kandels found nicotine dramatically enhanced the effect of cocaine. Denise's research suggests nicotine may have a similar effect in humans. She found most people who started using cocaine were also smoking. She also found the rate of cocaine dependence was highest among users who started using cocaine after having smoked cigarettes. Dependence was much lower among people who had begun using cocaine before they started smoking and among those who had never smoked more than 100 cigarettes.

While the typical e-cigarette user is described as a long-term smoker who is unable to stop smoking, e-cigarette use is increasing among teens and young adults, the Kandels note.

Study: Marijuana Withdrawal Symptoms Common in Teens Treated for Substance Use

A new study of teens treated at an outpatient substance abuse clinic found many showed symptoms of marijuana withdrawal. Marijuana was the substance used most often by 90 of the 127 teens in the study, HealthDay reports. Of those teens, 84 percent were considered to be dependent on marijuana, and about 40 percent experienced symptoms of withdrawal when they stopped using marijuana-a sign of drug dependence, the authors said.

"As more people are able to obtain and consume cannabis legally for medical and, in some states, recreational use, people are less likely to perceive it as addictive or harmful," study co-author John Kelly of Massachusetts General Hospital's Center for Addiction Medicine, said in a hospital news release." But research shows that cannabis use can have significant consequences, and we know that among adolescents it is second only to alcohol in rates of misuse."

Teens who had marijuana withdrawal symptoms were more likely to experience negative consequences, such as problems with school, work, relationships or finances. They were also more likely to meet criteria for marijuana dependence and mood disorders, the researchers wrote in the Journal of Addiction Medicine.

Teens who recognized and accepted that they had a substance abuse problem related to their marijuana use were more likely to make progress toward becoming abstinent, compared to those who did not think they had a problem.

Teens under age 17 who use marijuana every day are 60 percent less likely to graduate from high school, compared with their peers who have never used the drug, a new study finds.

Teen marijuana users are 18 times more likely to become dependent on the drug, seven times more likely to attempt suicide, and eight times more likely to use other illicit drugs later in life, the researchers report in The Lancet Psychiatry.

The researchers analyzed date from three previous studies that included almost 4,000 participants, according to CNN. The teens in the study were followed until they were 30. The more frequently teens used marijuana, the greater the risks of the drug, the study found.

An estimated 6.5 percent of high school seniors use marijuana daily or almost daily, according to the 2013 Monitoring the Future survey. The annual study looks at behaviors and attitudes of teens in eighth, 10th and 12th grades.