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."