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.