One child in every U.S. classroom has been diagnosed with attention-deficit/hyperactivity disorder (ADHD)--a number equal to 3 to 5 percent of all school-age children. To treat the condition, more than two million prescriptions for Ritalin--methylphenidate--are written each year.

Recently, the media has reported that Ritalin is a possible gateway drug to other stimulants, a conclusion that stems from research by Nadine Lambert, PhD, director of the school psychology program at the University of California at Berkeley. Lambert identified a relationship between both ADHD and the use of stimulants in childhood and dependence on tobacco and other stimulants as adults. She found that half the youngsters in her study who had been treated with Ritalin had become regular smokers by age 17. Only 30 percent of those never treated smoked regularly. And 36 percent of adult cigarette smokers treated with Ritalin as children, compared with 2 percent of those who never smoked or took prescription stimulants, were dependent on cocaine as adults.

"There are two possible explanations," Lambert says. "Kids self-medicate. They try a drug and it makes them feel better." Or, she posits, stimulants like Ritalin may sensitize kids to be susceptible to other stimulants. "This explanation has support from the animal literature," she notes.

In fact, a study, to be published in Neuropsychopharmacology (in press), at the Finch University of Health Sciences/Chicago Medical School found that adolescent rats given repeated doses of Ritalin, proportionate to doses for children, are more likely to self-administer cocaine as adults.

Lambert presented the ongoing 26-year study of more than 400 San Francisco Bay area children to the National Institutes of Health in 1998. She doesn't question Ritalin's value in treating children with ADHD. "There are benefits," she says. "But parents should be aware of the possible risks."

Other mental health professionals have raised the red flag about Ritalin as well. Peter Breggin, MD, director of the International Center for the Study of Psychiatry and Psychology and known for the books "Talking Back to Prozac" and more recently "Talking Back to Ritalin," told a U.S. House of Representatives education subcommittee last year that stimulants often foster future illicit drug use.

But others strongly disagree with that assumption. "There's no replicated evidence that treatment with Ritalin is associated with increased rates of substance abuse," says Brooke Molina, PhD, assistant professor of psychiatry and psychology at the University of Pittsburgh. In a 1999 poster presentation at the International Society for Research on Child and Adolescent Psychopathology, she reported that ADHD adolescents who were not currently treated with stimulants were more likely to drink heavily and have substance abuse problems. She is working on an ongoing longitudinal study of more than 300 adolescents and young adults diagnosed with ADHD in childhood.

Other studies have found that treatment of ADHD with stimulants reduces the likelihood of substance abuse. In 1999, Pediatrics published research by Joseph Biederman, MD, of Harvard Medical School, which concluded that ADHD boys treated with stimulants such as Ritalin are significantly less likely to abuse drugs and alcohol when they are older than ADHD boys who are not treated.

Meanwhile, Russell Barkley, PhD, professor of psychiatry and neurology at the University of Massachusetts Medical School, has found that stimulant treatment in childhood is unrelated to drug use in adolescence. Steven Evans, PhD, associate professor of psychology at James Madison University, says claims that Ritalin is a gateway drug are somewhat contradictory because "it's the characteristics of children with ADHD that are most likely to lead them to substance abuse, not the drug," he says.

William E. Pelham Jr., PhD, professor of psychology, psychiatry and pediatrics at State University of New York-Buffalo and Molina's research collaborator, agrees. "It's not the Ritalin, it's the ADHD that might lead to smoking or substance abuse."

Although stimulant treatments for ADHD will continue to incite debate, the finding that treatment with Ritalin may cause children to abuse stimulants as adults seems inconclusive.

"At this point, there's only one study and it hasn't been replicated," says Barkley. "And everyone knows, replication is the hardest science."