Marijuana arrests account for almost half of all drug arrests in the United States, which spends $4 billion a year to catch, prosecute and incarcerate offenders, according to a report released on Tuesday.

“Since 1990, there have been 6.2 million arrests for marijuana possession and an additional one million for marijuana trafficking. As of 2002, marijuana arrests comprised 45 percent of all drug arrests,” the report by The Sentencing Project said.

The Washington think tank, which promotes alternatives to imprisonment, said daily use of the drug by high school seniors nearly tripled to 6 percent from 2.2 percent during the years 1990 to 2002.

Meanwhile, the street price of the drug has fallen in real terms and its purity has increased.

It also found that of the 734,000 marijuana arrests in 2000, only 6 percent resulted in a felony conviction. Marijuana arrests more than doubled, from 327,000 to 697,000 from 1990 to 2002, while arrests for other drugs rose by only 10 percent.

“The ‘war on drugs’ has been transformed into a ‘war on marijuana’ through dramatic shifts in law enforcement policies and practices,” said Ryan King, co-author of the report.

He said arresting such large numbers at an annual cost of $4 billion was a poor investment in public safety and diverted resources from more serious crime problems.

Meanwhile, the U.S. government’s Substance Abuse and Mental Health Services Administration said in a report released on Tuesday that adults who first used marijuana before the age of 12 were twice as likely to suffer from mental illness later in life than those who used the drug at age 18 or older.

The data came from an annual survey on drug use, which found that 43 percent of U.S. adults - almost 91 million people - reported using marijuana at least once in their lives.

“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 Walters.

Jennifer Devallance of the White House drug policy office said it was inaccurate to portray the “war on drugs” as focused on a single substance.

“However, marijuana is the drug most abused in this country and the single largest source of treatment need, so it is appropriate to focus effort and attention on marijuana,” Devallance said.

She disputed the report’s contention that marijuana use had increased, citing figures from a survey of high school teens showing it was down 18 percent over the past three years.