Thirty-five restaurants across China have been found illegally using opium poppies as a seasoning, officials have revealed.
Five are being prosecuted while 30 are under investigation, the China Food and Drug Administration said.
Local authorities are being urged to help investigators find the sources of the poppies, the China Daily reported.
Poppy powder, which contains low amounts of opiates, is banned as a food additive in China.
However, restaurants have previously been caught using it.
In 2012, seven restaurants in Ningxia province were closed for using opium poppies and in Guizhou province in 2004 authorities shut down 215 establishments for similar offences.
One of the businesses affected by the latest crackdown is reported to be the popular Huda Restaurant chain in Beijing.
General manager Hu Ling confirmed the company was under investigation and said it may have unwittingly bought seasoning contaminated with opiates. She declined to comment further.
China has been hit by a series of food scandals in recent years.
In 2014 a Shanghai-based supplier was found to have sold unsanitary and expired chicken meat to food chains including KFC, Starbucks and McDonald's.
In 2008, six children died and more than 300,000 were made ill from milk powder contaminated with melamine, an industrial chemical used to make plastics and fertiliser.