A groundbreaking California program designed to get drug abusers the medical help they need and alleviate strain on the state's prison system faced challenges in its first six months, according to a new study.

Researchers from the University of California at Los Angeles looked at how drug abusers fared since the state's Substance Abuse and Crime Prevention Act took effect in July 2001. In 2000, voters backed a plan to send nonviolent drug offenders to rehab instead of prison. Supporters said it would be more effective and less expensive than incarceration.

The study published on Friday found that offenders in rehabilitation were 48 percent more likely to be arrested for a drug offense within a year of starting rehab than drug users who were on parole or probation.

David Farabee, the study's lead author and a research scientist at UCLA, said he also "found that abusers with the most severe problems were unlikely to get treatment in a residential program."

A residential program is the most effective way to treat patients with the most severe drug problems, he said. Drug abusers sent to outpatient programs are more likely to be re-arrested, the study found.

Farabee said the study was not trying to judge the success or failure of the program, but where improvements could be made.

The study looked only at the early months of the program -- July 1 through Dec. 31, 2001 -- when counties were still grappling with how to implement it.

An influx of drug abusers into treatment programs, coupled with severe budget constraints in California pose difficult challenges for the program administrators, Farabee said.

Del Sayles-Owen, deputy director for criminal justice collaboration at California's Alcohol and Drug Program said the report is not an accurate evaluation because it was "a snapshot in time -- following just 688 cases in 13 counties for the first six months."

"We know that procedures have evolved since the early days," she said.

Sayles-Owen said more recent data shows more abusers are completing treatment programs.

The study appears in the November issue of Criminology & Public Policy.