Three Republican senators want the Trump administration to tighten federal limits on kratom, a plant with opioid-like effects that’s become a popular ingredient in pain-relief and mood booster products sold in gas stations nationwide. Sens. Chuck Grassley of Iowa, Mike Lee of Utah and Susan Collins of Maine urged Drug Enforcement Administration Administrator Terrance Cole and acting Food and Drug Administration Commissioner Kyle Diamantas to pursue permanent controls on kratom and its derivatives in a letter sent Wednesday.

“We encourage DEA, in consultation with the Food and Drug Administration (FDA), to consider scheduling the entire kratom plant,” they wrote. The FDA and the DEA didn’t immediately respond to requests for comment.

The senators pressed the DEA to finalize its July proposed emergency scheduling order for 7-hydroxymitragynine, a chemical compound found in the kratom plant that has been commonly found in the products. The Department of Health and Human Services has linked 7-OH-containing products to a rise in overdose risks and emergency room visits and has pushed for it to be classified as a controlled substance. “It is imperative that the final emergency scheduling order for 7-OH be expeditiously promulgated,” the lawmakers said.

They also called for the permanent scheduling of MGM-15, MGM-16 and mitragynine pseudoindoxyl — semisynthetic derivatives related to compounds found in kratom that have been found in certain wellness products. The Justice Department temporarily moved the three substances to Schedule I of the Controlled Substances Act — the strictest drug classification — in August. “Permanently placing all levels of these synthetic compounds into Schedule I of the Controlled Substances Act is an essential and necessary action to ensure the safety of kratom consumers and to keep products with compounds even more dangerous than 7-OH from widespread availability,” they wrote.

The lawmakers cited increasing poison-center reports and thousands of suspected nonfatal overdoses involving kratom or related compounds plus testimonies from “constituents and concerned parents who have lost loved ones” in their letter. The DEA pitched placing kratom on Schedule I in 2016 but withdrew the proposal after facing significant backlash from scientists and some lawmakers. There’s been renewed attention to kratom after federal prosecutors announced criminal charges earlier this month against smoke shop employees following the suspected overdose deaths of two University of Mississippi students.

Investigators reported finding pills that were either kratom or synthetic kratom at both crime scenes. Although the Trump administration has sought to rein in 7-OH and other kratom-related compounds, reporting from media outlets has chronicled a supportive relationship between the kratom industry and Health Secretary Robert F. Kennedy Jr.’s Make America Healthy Again movement. This year, the kratom supplement company Botanic Tonics donated $1 million to MAHA PAC, a political action committee tied to the Kennedy-backed coalition.

The kratom industry quickly criticized the senators’ letter. Mac Haddow, a senior fellow on public policy at the American Kratom Association — an advocacy group seeking to preserve legal access to certain kratom products — told POLITICO on Wednesday that the letter should be “retracted and clarified.”“We do have a big hill to climb here to educate Congress,” Haddow said. “This letter undermines accuracy, good science and the right public policy.”The lawmakers requested a response by Oct. 15.