BEIJING — It is no secret that fans of Chinese food often find it addictive. They speak longingly of the numbing spice of Sichuan peppercorn or the sour herbs found in Yunnan cooking.

At least 36 restaurants in China tried to take that addictive quality one step further, and now have found themselves on the wrong side of the law. They were investigated in the past week by the authorities for adding poppy capsules and other illegal ingredients to food, according to a report on Friday by Xinhua, the state-run news agency.

The poppy capsule is made from the dried pericarp of the ripe fruit of an opium poppy plant, and it has more than 20 types of alkaloids, including those found in morphine and cocaine, the report said. It is used in the Chinese medicine industry, but it is illegal in cooking.

Transporting, buying, selling, storing or using poppy capsules in food can result in up to 15 days in detention or a fine of up to 3,000 renminbi, or $455.