Forget Colorado, stoners. The real frontier of narcotic edibles is in Shaanxi province, China. A restaurant owner there just confessed to police that to keep customers coming back, he had infused his noodles with 2 kilograms (4.4 pounds) of pulverized poppy buds—which can contain narcotics like morphine and codeine—that he bought in August for 600 yuan ($98).

Apparently, it worked; the restaurant boss said customer numbers leapt (link in Chinese) after he started using his “special” seasoning. Chinese authorities say doses were enough to addict frequent diners, reports the South China Morning Post (paywall). Police launched an investigation only after one of the restaurant’s repeat customers tested positive for opiates in a routine urine screen.

But Zhang, the shop owner, wasn’t the first Chinese restaurateur to strike upon this idea—not by a long shot. An investigative report in 2011 found that illegal poppy products are available in Shaanxi markets—with restaurant owners being the prime customers.

In the opiate dining market, however, Shanghai gives Shaanxi a run for its money. Just last May, a Shanghai restaurant owner was sentenced to 10 months in jail for zesting up soups with morphine. In March 2014, police jailed Shanghai restaurant owners for using Narceine, another poppy-shell opiate, to dope up a famous crayfish dish called xiaolongxia. In 2010, three Shanghai hotpot restaurants (link in Chinese) were shuttered for adding opiates.

Restaurant owners all over China have long embraced this customer retention trick. Last year, two restaurants in the southern metropolis of Guangzhou were caught adding pulverized poppies to their food. The prior year, seven restaurants in Ningxia were found to be dousing their hotpot soups with morphine. A Sichuan restaurant has repeatedly been found to feature codeine as a “secret ingredient” (link in Chinese). In Guizhou province in 2004, police busted 215 restaurants for morphine-laced hotspots.

UN Office on Drugs and Crime Poppy cultivation is on the rise in the Golden Triangle.

These are only the ones that have been caught, amid a broader spate of food-safety scandals dogging the Chinese government and inspiring public outrage. The source of the opium supply isn’t clear, but western China abuts the Golden Triangle—the prime poppy-cultivation areas of Myanmar, Laos, and Vietnam. Its first noted use in China was as a surgical anesthetic as early as 220 AD, and the commodity played a major role in China’s struggle against the British Empire in the 1800s (its double defeats in the Opium Wars are still a national sore spot).

Note that while all parts of the poppy plant contain some level of opiates, the seeds common on bagels and muffins aren’t typically used in large enough quantities to be psychoactive (or, as popularized on Seinfeld, to cause drug test failures).

Not everyone is as lucky as a Seinfeld character. The 26-year-old diner whose urine test exposed the noodle shop’s secret is still in prison for drug use—even though further police testing suggested he tested positive as a result of the poppy-laced noodles. “Whether it’s through self-inflicted drug use or unwitting food consumption, it’s still drug use,” says local police chief Ma Yubin told the Xi’an Evening News. ”The law doesn’t draw a sharp distinction between the two.”