Deftly, Ms. Tsukioka, a 29-year-old experimental fashion designer, lifted a flap on her skirt to reveal a large sheet of cloth printed in bright red with a soft drink logo partly visible. By holding the sheet open and stepping to the side of the road, she showed how a woman walking alone could elude pursuers — by disguising herself as a vending machine.

In a city once synonymous with decadence, a new drug menace has emerged in Shanghai. Recreational users are bingeing on ketamine, cocaine and methamphetamine to keep them playing into the early hours.

This time it is not the young and reckless fuelling late-night tours of nightclubs. Instead, says a state newspaper, retired and middle-aged residents are turning to drugs to stay awake through marathon sessions of mahjong.

Zheng Yuqing, spokesman for the Shanghai anti-drug commission, warned of a "significant increase" in cases among 40- to 60-year-olds. "The drug-taking mostly occurs among groups in card rooms, a place popular among the elderly where they can get together and play cards. The addicts are often friends who have known each other for years," he told China Daily.

The newspaper said many took cocaine and other stimulants during card-playing and mahjong sessions that often lasted all night. The games frequently involve gambling, which is illegal on the mainland.

"Most of the older victims are unemployed or retired and so have plenty of spare time. They are often not well-educated and have little awareness of the harm caused by taking drugs," Zheng added.

While most users are teenagers or youthful professionals, the proportion of drug addicts who are under 35 has declined from 77% nationwide in 2001 to just under 60% last year, the paper reported.

"More and more middle-aged and older people take drugs because they feel lonely and empty after retiring or losing their jobs," said Li Luyan, secretary-general of Shanghai Sports Association for the Aged. "Their children and society in general should show more care to these people to keep them away from drugs … they need more activities, such as sports, to fill their spare time."

Drug abuse among mahjong players will be disappointing to sports officials, who have tried to clean up the game's image in recent years. They even asked for advice from enthusiasts in the United States, where the game is popular with middle-class women.

"Initially the game was played in opium dens," pointed out Ruth Unger, president of the New York-based National Mah Jongg League and one of those who advised Chinese officials. "[More recently] there was a complete turnaround and they were trying to promote 'healthy mahjong'."

Unger said the game itself was addictive, which helps to explain why it can tax the health of even youthful participants.

Last year a 30-year-old man in Guizhou province slipped into a coma after playing for 32 hours non-stop, state media reported.