Thousands of prisoners wait months to enter drug education or rehabilitation programs, according to a report by the Government Accountability Office (GAO).

The delay is caused by staff shortages and limited resources, USA Today reports. Drug offenders represent the largest category of prisoners in the federal prison system, the article notes.

In 2011, more than 51,000 inmates were on waiting lists for basic drug education programs, some for up to three months. A total of 31,803 inmates were enrolled in such programs last year, the report states.

Inmates who complete the Federal Bureau of Prisons' Residential Drug Abuse Program receive a sentence reduction of one year. Waiting lists for the program were so long last year that only one-quarter of graduates entered the program with at least a year left on their prison terms.

"These are important programs, because so many people come into the system with substance abuse problems," David Maurer, primary author of the GAO review, told the newspaper. "These programs can help in the whole re-entry process."

According to Federal Bureau of Prisons spokesman Ed Ross, the number of inmates on waiting lists for the programs, and the time spent waiting for treatment, has begun to decrease.

"To the extent the budget allows, we will continue to add treatment staff to meet the needs of the increasing inmate population, and in the future, we expect to reduce the amount of time an inmate is wait-listed for treatment," he said. "Reducing the time spent waiting to enter treatment will allow for longer sentence reductions at the back end for non-violent eligible inmates."