BEIJING — Fu Lixin, emotionally exhausted from caring for her sick mother, needed a little pick-me-up. A friend offered her a “special cigarette” — one laced with methamphetamine — and Ms. Fu happily inhaled.

The next day, three policemen showed up at her door. “They asked me to urinate in a cup,” she said. “My friend had been arrested and turned me in. It was a drug test. I failed on the spot.”

Although she said it was her first time smoking meth, Ms. Fu, 41, was promptly sent to one of China’s compulsory drug rehabilitation centers. The minimum stay is two years, and life is an unremitting gantlet of physical abuse and forced labor without any drug treatment, according to former inmates and substance abuse professionals.

“It was a hell I’m still trying to recover from,” she said.

According to the United Nations, as many as a half million Chinese citizens are held at these centers at any given time. Detentions are meted out by the police without trials, judges or appeals. Created in 2008 as part of a reform effort to grapple with the country’s growing narcotics problem, the centers, lawyers and drug experts say, have become de facto penal colonies where inmates are sent to factories and farms, fed substandard food and denied basic medical care.

“They call them detoxification centers, but everyone knows that detox takes a few days, not two years,” said Joseph Amon, an epidemiologist with Human Rights Watch in New York. “The basic concept is inhumane and flawed.”

Recently, Human Rights Watch issued a report on the drug rehabilitation system that replaced the Communist Party’s previous approach of sending addicts to labor camps, where they would toil alongside thieves, prostitutes and political dissidents.

The report, titled “Darkness Knows No Limits,” calls on the government to immediately shut down the detention centers.

Under the Anti-Drug Law of 2008, drug offenders were to be sent to professionally staffed detox  facilities and then released to community- based rehabilitation centers.

But Wang Xiaoguang, the vice director of Daytop, an American-affiliated drug-treatment residence in Yunnan Province, said the government detox centers were little more than business ventures run by the police. Han Wei, 38, a recovering heroin addict who was released from a Beijing detention center in October, said the guards would use electric prods on the recalcitrant. “At least they’d give us helmets so we wouldn’t injure our heads during convulsions,” he said.

Despite the deprivations, Mr. Han, a former nightclub owner, said his two-year sentence achieved the desired goal: it persuaded him to kick a habit he began in 1998. “I’m never going back,” he said.

Zhang Wenjun, who runs Guiding Star, an organization that helps recovering addicts, said such determination was most often fleeting. At least 98 percent of those who leave the drug detention system relapse within a few years, he said.

“What the government doesn’t realize is that this is a disease that needs to be treated, not punished,” said Mr. Zhang, 42.

In some ways, he said, the stigma of addiction is as crippling as the lure of the next fix. Those arrested for drug offenses are branded addicts on their national identification cards, which makes applying for jobs and welfare benefits acts of futility. And because the local police are notified when former offenders check into hotels, traveling often involves impromptu urine tests and the possibility of humiliation in front of colleagues.

“In China, to be a drug addict is to be an enemy of the government,” Mr. Zhang said.