The China Food and Drug Administration said in a statement this week that a total of 35 restaurants were under investigation and five of those had already been prosecuted.
The regulator said the poppy derivatives it detected included morphine and codeine and called on local authorities to step up their monitoring efforts. The additive has been banned since 2013.
Cooks sprinkling powdered poppy on their dishes isn't uncommon in China although it's unclear whether it can make a restaurant's food genuinely addictive.
In 2014, a noodle vendor was detained for 10 days after admitting adding powdered poppy plant to his dishes to keep customers coming back.
The case only came to light after police stopped a vehicle driven by a 26-year-old man and tested him for drugs not long after he had consumed a bowl of the noodles.
Recently, China has been rocked by a number of stomach-churning food safety scandals. The most high-profile case was in 2014 when a supplier was found to have provided tainted meat to McDonalds and other restaurant chains.
