File Photo: Plastic bags of fentanyl are displayed on a table at the U.S. Customs and Border Protection area at the International Mail Facility at O'Hare International Airport in Chicago, Illinois, U.S. Nov. 29, 2017. (Xinhua/REUTERS)

SAN FRANCISCO, Dec. 5 (Xinhua) -- Wholesale dealers that sell fentanyl are driving the rise of fentanyl in the U.S. drug market because of heroin supply shocks and shortages, a study of University of California San Francisco (UCSF) said Tuesday.

The findings of the study by UCSF researchers, which were published in the journal Addiction on Tuesday, said fentanyl is the easiest and cheapest drug for manufacturers and traffickers to produce, usually at a rate fast enough to meet the rapidly rising demand for opioids in the United States.

Fentanyl, a powerful synthetic opioid that is 30 to 40 times stronger than heroin, caused nearly 29,000 overdose deaths in the United States, said the study.

With a more powerful effect, fentanyl is much cheaper to produce than heroin, which is derived from poppy opium, and it is usually sold deceptively as heroin or other brand name prescription drugs, the study said.

"The dealers selling fentanyl directly to the users often don't know what's in it. Not only is this particularly dangerous, but it also means penalizing low level dealers isn't going to make any difference in the fentanyl poisoning epidemic," said Sarah Mars, the first author of the study and a researcher in the Department of Family and Community Medicine at UCSF.

The UCSF researchers found that most of the illicit fentanyl has been in the Northeast and Midwest, which reported a high death rate.

They said many drug users and street-level dealers did not have enough accurate information about the drugs, and it's impossible for them to determine which synthetic opioid is present or how potent it is before they use it.

As prescription opioids are becoming harder to obtain in the United States, which created heroin shortages or "supply shocks," fentanyl is an easy substitute for heroin because of its cheaper production cost. The drug is entirely lab-made and can be produced all year round.

There are "significant incentives" for wholesale suppliers to shift to fentanyl, either partially or completely, even though it often causes users to overdose, according to the study.