Perhaps the most shocking example of China’s huge drug trade exploded into the public consciousness in December 2013, when 3,000 paramilitary police officers raided a small village on the coast of Guangdong Province and arrested 182 people, including the former party secretary and 13 other officials. Nearly three tons of meth were seized from the village. “Meth is popular because any illegal lab or factory in the mainland can make it,” said Lu Lin, the director of the China Medical Dependency Research Institute at Peking University in Beijing.

Some of the key ingredients in meth are derived from the herb ephedra sinica, known as ma huang in Mandarin, a staple of traditional Chinese medicine used for treating colds and coughs. Experts say much of the country’s meth is produced in southern China, though the authorities prefer to blame Southeast Asian countries like Myanmar and Laos. Consistently absent from their accusations is North Korea, a close ally that some experts believe churns out vast quantities of meth trafficked into China’s northeast.

For years, Beijing residents have wondered how dealers were able to sell their wares so openly near a police station in the Sanlitun district, home to many embassies, bars, and restaurants popular with expatriates. A crackdown scattered the men last spring, but during a recent stroll through the neighborhood, it was clear they have not gone far.

Sun Zhongwei, a former narcotics officer turned lawyer, dismissed the suggestion that the dealers were officially tolerated. “If Chinese police had spotted them, they’d have been arrested,” he said. “It’s impossible for the police to see them and not act upon it. That would be considered an act of negligence.”

But drug users in China say the police operate in a bureaucracy programmed to follow orders from above. In some cities, the police allow dealers to operate undisturbed — until they need to fill a quota, according to He Mukun, a former addict and drug counselor in Yunnan. Mr. He said the police in Yunan rarely arrested drug dealers, preferring to use them as informants during crackdowns. “The police think, ‘In the future, when my boss gives me an assignment to catch drug users, what happens if I can’t find any?’ ” he said. “But if a cop knows a drug seller, he can just ask for a bunch of names. You get huge numbers that way.”

Indeed, the eye-popping statistics from the Ministry of Public Security appear intended to impress: In a five-month crackdown last year, the police were said to have “totally uncovered” 50,827 drug cases, arrested 56,989 suspects and seized 26.5 tons of drugs, an increase in seizures of 126.8 percent over the same period a year earlier.