Four types of synthetic opioids, including an elephant tranquilizer, are to be added to China's list of controlled substances, as authorities crack down on a new generation of illegal narcotics.

Starting March 1, four fentanyl-based medications will be put on the List of Nonpharmaceutical Narcotic Drugs and Psychotropic Drugs Under Control, taking the total number of controlled substances to 134, the Ministry of Public Security said on Thursday.

Fentanyl is a synthetic opioid that is about 10,000 times more powerful than morphine and is identified as a new psychoactive substance, China's so-called third generation of drugs.

It is used legitimately as an anesthetic in hospitals or for long-term pain management, such as in the form of prescription patches for late-stage cancer patients, according to Hua Zhendong, deputy director of the ministry's drug testing laboratory.

He said the drug is also used by veterinarians to sedate animals and as a general anesthetic. Fentanyl has become widely used as an illicit street drug in the United States and Europe in the past decade, he added.

Fentanyl can be deadly for an adult in a 2-milligram dose, according to the ministry's lab. The drug has been blamed for thousands of accidental overdose deaths caused when it was mixed with heroin by drug dealers, according to health authorities.

The substance drew wide public attention after Prince, the US musician, died in April at his home of an accidental overdose of fentanyl.

"More than 10 people suspected of producing new psychoactive substances have been detained, with more than 800 kilograms of the substances seized since 2016," said Yu Haibin, head of the department charged with clamping down on the production of controlled substances at the ministry.

Among them were chemistry professors and others with knowledge of chemicals.

Wang Hua, who studied chemistry at Peking University, quit his pharmaceutical company job and started his own business in 2014. He was caught in June exporting 630 kg of methcathinone hidden in LED tubes.

"One of the challenges we are facing is a number of drug deals are done online, making it difficult for police officers to track and gather evidence," Yu said.

Ministry officials said that 23 types of fentanyl have been included on the list of controlled substances in China. Only three of them are legitimately used in hospitals and pharmacies.

zhang_yi@chinadaily.com.cn