WASHINGTON (Reuters) - Lawmakers on the U.S. Senate Judiciary Committee have urged the Trump administration to conduct a scientific review of a Justice Department-backed bill to classify all illicit chemical knockoffs of the potent painkiller fentanyl in the same legal category as heroin.

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. November 29, 2017. REUTERS/Joshua Lott/File Photo

The sweeping legislation may “deter valid, critical medical research aimed at responses to the opioid crisis,” the senators said in a July 10 letter to Department of Health and Human Services (HHS) Secretary Alex Azar seen by Reuters on Thursday.

Lawmakers and health officials have said fentanyl, which is about 100 times more potent than morphine, has fueled the opioid overdose epidemic.

As prescribed by physicians, fentanyl is classified as a Schedule II drug, meaning it is highly addictive but has a medicinal purpose, typically to treat intense cancer pain.

But chemists primarily in China have created numerous slightly altered versions of the drug, known as “analogues,” that have hit the U.S. streets.

If the draft bill is passed by Congress, it would place all illicit fentanyl analogues in Schedule 1, along with heroin, would means that they are addictive, have no medicinal purpose and are effectively banned.

The legislation is designed to help prosecutors keep pace with criminals who churn out chemically tweaked fentanyl analogues to evade strict Schedule I regulations.

But scientific experts, including some within HHS, contend that automatically placing all analogues into Schedule 1 could stifle research to combat the opioid crisis. They argue that Drug Enforcement Administration (DEA) regulations to win approval for such research are so onerous that they will deter many scientists from applying for needed waivers.

In their letter to Azar, senators said the administration had “not adequately consulted with public health agencies” about the impact of classifying all fentanyl analogues as Schedule I.

“We are concerned that the failure to engage necessary health experts vests far too much authority to a law enforcement agency,” they wrote, adding that it could also “deter valid, critical medical research.”

The letter was signed by Democratic Senators Richard Durbin, Sheldon Whitehouse, Amy Klobuchar, Christopher Coons, Mazie Hirono, Cory Booker and Kamala Harris, as well as Republican Senator Mike Lee.

A spokesperson for HHS confirmed receiving the letter and said all congressional inquiries are taken seriously.

The DEA did not have any immediate comment.

The letter came after Reuters this week reported about an ongoing interagency dispute between an office within HHS and the DEA over the proposed fentanyl analog legislation.

In a June 20 closed-door briefing with Senate Judiciary Committee staffers, an official from the National Institute on Drug Abuse warned that the bill as drafted could create regulatory hurdles that will make it too hard for scientists to research possible medical benefits of fentanyl analogues.

Such benefits could include antidotes to overdoses, or the creation of pain killers without addictive properties.

Normally, the DEA and the U.S. Food and Drug Administration review chemical compounds individually to assign each one a controlled substance classification, with the FDA determining if such “scheduling” decisions are scientifically valid.

The draft bill, introduced by Republican Senator Ron Johnson and Republican Representative Jim Sensenbrenner, would cut the FDA out of that process.