Updated November 21, 2014.

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Parents and community leaders need to be aware that if they have visible drug activity taking place in their neighborhoods, it greatly increases the chance that the local teens will report using illicit drugs. Teens who see "open" drug sales in their neighborhoods "almost every day" are 11 times more likely to report using drugs compared to teens with no visible drug activity.

How does that work? It's a complicated relationship, the experts say.

Peer Attitudes Are Important

Most research on the cause of early-onset substance abuse has focused on individual, family and peer factors. The few studies that have looked at how neighborhoods influenced drug use by young people have focused on crime and socio-demographic characteristics.

Some of those previous studies found that children are less likely to use drugs when their peers express disapproval for illicit drugs. On the other hand, if their peers approve of drug use, or use drugs themselves, teens are more likely to use them also.

Open Drug Activity Reduces Disapproval

Now, a New York University's Center for Drug Use and HIV Research study has found that the more teens see drug activity going on in their neighborhoods, the less likely they are to express disapproval.

As the disapproval rate for illicit drug use decreases, the more teens are likely to try drugs themselves, the researchers found.

Dr. Dustin T. Duncan and associates used data from 10,050 high school seniors who completely answered questions on the Monitoring the Future survey about neighborhood drug sales and peer disapproval.

Greater Quantities, Harder Drugs

The researchers not only looked at a link between neighborhood drug sales and drug use, but also tried to determine if their was a relationship between the perceived frequency of drug sales and peer disapproval.

The study found a strong relationship between neighborhood drug sales and drug use. Not only were high school seniors more likely to do illicit drugs in those neighborhoods, they were more likely to do greater quantities of drugs and "harder" drugs.

For the first time, the researchers found an association between "open" drug sales in the neighborhood and a decreased peer disapproval of marijuana and cocaine.

Drug Use 'Normalized' by Open Sales

Students in the survey who reported seeing drug sales "almost every day," were also less like to report that their friends strongly disapproved of marijuana. The researchers believe that seeing frequent drug sales acted to "normalize" drug use and lead to lower disapproval.

The lower disapproval helps explain increased drug use among this group because, the researchers said, their previous studies found that disapproval or stigma toward drug use is a protective factor against teen use of marijuana, cocaine and ecstasy.

What Can Communities Do?

Duncan and his research team believe a reduction in "open" drug sales can reduce peer approval and subsequently reduce illicit drug use.

The scientists point to criminology and public health studies that show drug-related crimes can be reduced by modifying the social and physical neighborhood environment through various strategies. Problem-oriented policing and installation of surveillance cameras can be used to reduce "open" neighborhood drug sales.

Controls Needed in Decriminalized Areas

To reduce early illicit drug use among children, open drug sales need to be controlled, the researchers said, even in areas where recreational use of drugs has been decriminalized or legalized.

Even in states where marijuana is legal, it's not legal for teens. Reducing open drug sales in neighborhoods could reduce peer approval and subsequently reduce drug use, Duncan said.

^{Duncan, DT et al. "Perceived neighborhood illicit drug selling, peer illicit drug disapproval and illicit drug use among U.S. high school seniors." Substance Abuse Treatment, Prevention, and Policy September 2014.}