The number of teens abusing drugs is lower than it's been since the 1990s, according to a national survey. "The most important finding is the rate of drug use is at the lowest levels we've ever seen," says Dr. Nora Volkow, the director of the National Institute on Drug Abuse, which funded the study. But there are a couple of key exceptions. "[One] is marijuana. It hasn't gone up, like in older populations, but it hasn't gone down, and it remains worrisome," Volkow says.