Children who use marijuana before age 12 are twice as likely to later develop serious mental illness as those who don't try the drug until they're 18, according to a federal report released Tuesday.

Bush administration officials pointed to the study as growing evidence that smoking marijuana may cause mental illnesses - including depression, schizophrenia, and suicide attempts - in some people.

But while the association of drug abuse and addiction with mental illness is well known, a causal connection between marijuana smoking and psychiatric disorders is not clear, other experts say.

In Tuesday's study, 21 percent of people who reported first using marijuana before age 12 also reported that they later went on to develop signs or symptoms of a serious mental illness. Those who said they used the drug only after age 18 had a 10.5 percent chance of reporting similar problems.

The study was based on federal drug use data culled in 2002 and 2003. Other past studies publicized by federal officials Tuesday also point to a connection between marijuana use and the development of mental problems later on.

"New research being conducted here and abroad illustrates that marijuana use, particularly during the teen years, can lead to depression, thoughts of suicide, and schizophrenia," said White House Drug Czar John P. Walters. "This press conference is a public health warning."

Another study highlighted by officials, published in 2001, suggested that people who were not depressed but used marijuana were four times more likely to develop depression years later than those who never used the drug.

Is Marijuana Use a Way to 'Self-Medicate' Mental Illness?

Researchers have long observed a connection between drug use and mental illness. Many studies show the simultaneous occurrence of mental illness and substance abuse. People with mental illnesses are also known to use drugs to lessen their symptoms, a phenomenon psychiatrists refer to as "self-medicating."

But federal officials and some researchers say evidence is accumulating that shows that marijuana can actually cause serious mental illnesses in otherwise well people.

According to the National Institute on Drug Abuse (NIDA), depression, anxiety, and personality disturbances have been associated with marijuana use. However, the NIDA says it is not known whether marijuana use is an attempt to self-medicate an already existing mental health problem, or whether marijuana use leads to mental disorders (or both).

"The evidence is collectively indicating that there is a causal connection," says Neil McKeganey, PhD, professor of drug misuse at the University of Glasgow in Scotland.

McKeganey notes that scientists have not yet uncovered evidence linking marijuana use to the brain changes routinely seen in people who suffer from mental illness. "If we wait until we understand that mechanism, we will lose thousands of young people," he says.

But Paul P. Casadonte, MD, a psychiatrist and associate clinical professor at New York University, cautions in an interview that research is not yet strong enough to show a causal link between marijuana use and serious mental disorders. He suggests that such claims by Walters and other administration officials were intended to further the Bush administration's efforts to quell young peoples' marijuana use.

"That's dangerous territory. It's politics more than science at this point," says Casadonte, who is also director of substance abuse treatment programs at New York Department of Veterans Affairs Medical Center.

Casadonte warns that Tuesday's study of early marijuana use does not necessarily prove that smoking at a young age leads directly to later illness. "We do know that the younger you start, the more likely that there's something mentally wrong with you to begin with. Marijuana has more of an addiction potential than most people want to believe," he says. "But basically we just don't have the science" to claim a causal link with mental illness."

Federal officials remain alarmed at high rates of marijuana use in younger and younger U.S. children. According to the NIDA, marijuana is the most commonly used illicit drug in the U.S.; nearly 53 percent of Americans who say they've used the drug say they tried it for the first time before age 17.

According to data from the CDC, one-tenth of students nationwide had tried marijuana for the first time before age 13. Overall, the rates were higher in males (13 percent) than in females (6 percent).

Officials are preparing to launch a national campaign using newspaper and magazine advertisements to draw parents' attention to a link between marijuana use and mental illness.

Sources: John P. Walters, director, White House Office on Drug Control Policy. Neil McKeganey, PhD, professor of drug misuse, University of Glasgow, Scotland. Paul P. Casadonte, director, Substance Abuse Treatment Programs, New York Department of Veterans Affairs Medical Center. National Institute on Drug Abuse. CDC.