More than 10 senators are calling on the Drug Enforcement Administration to suspend plans to make the herbal supplement kratom a Schedule I substance until Congress can review the agency’s decision. The requests, sent this week to acting DEA Administrator Chuck Rosenberg, build on a similar move in the House.

“Given the long reported history of Kratom use, coupled with the public’s sentiment that it is a safe alternative to prescription opioids, we believe using the regular review process would provide for a much-needed discussion among all stakeholders,” reads a letter written by Sen. Orrin Hatch (R-Utah) and signed by Sens. Mike Lee (R-Utah), Mark Kirk (R-Ill.), Angus King (I-Maine), Michael Bennet (D-Colo.), Bernie Sanders (I-Vt.), Thom Tillis (R-N.C.), Ron Wyden (D-Ore.) and Mark Warner (D-Va.).

In a notice published in the Federal Register at the end of August, the DEA claimed that kratom, an herb made from the leaves of Mitragyna speciosa, a Southeast Asian tree related to coffee, poses an “imminent hazard to the public safety” and should therefore be placed in Schedule I as soon as the end of September. Drugs in this category include heroin and LSD and are considered to have no known medical benefit and a high potential for abuse.

A separate letter, sent to Rosenberg on Thursday by Sen. Cory Booker (D-N.J.) and signed by Sens. Kirsten Gillibrand (D-N.Y.) and Wyden, also calls out the DEA for moving so swiftly to ban kratom.

“We are concerned that the 30-day comment period for such a proposed regulatory action is not a sufficient amount of time on a drug that, according to recent scientific studies, may be an effective substance to help combat the opioid epidemic,” the senators write.

Anecdotal evidence and emerging scientific research suggests kratom can be used to treat a variety of conditions, including chronic pain, anxiety, depression and opioid addiction. Booker’s letter notes that a group of 11 scientists from “well-respected research institutions in the U.S.” wrote Congress earlier this month to express “grave concern” about the DEA’s proposed action.

Kratom works by activating opioid receptors in the brain. And while most narcotic opioids have sedative qualities that can lead to respiratory depression and death when taken in excess, kratom is a mild stimulant in low to moderate doses.

But in justifying its intent to ban kratom, the DEA focused exclusively on isolated reports of abuse and harm linked to the herb, which critics say are misleading outliers. The agency also notes that because the herbal supplement hasn’t been approved as a medicine under any federal regulatory process, it doesn’t accept any claims about its benefits.

The federal government offered no period for public comment ahead of the announcement, and kratom supporters said the news took them by surprise. The ban was initially scheduled to take place as early as Friday, but amid widespread backlash from users, drug policy reform advocates and now members of Congress, such immediate action appears unlikely.

In Hatch’s letter, the senators say that scheduling decisions like this must allow for the “full engagement of consumers, researchers, health professionals, law enforcement officials, and other stakeholders.” They also rebuff the DEA for making a unilateral push to enact a prohibition on an herb.

“The Congress granted emergency scheduling authority to the DEA based on the need for law enforcement interdiction of new and previously unknown illegal synthetic street drugs that result in injuries and death,” they write. “The use of this emergency authority for a natural substance is unprecedented, so it is important to determine whether the circumstances here necessitate a jump to Schedule I.”

The DEA has the power to make emergency scheduling decisions under the Controlled Substances Act. In 2012, Congress enhanced that authority as part of legislation meant to address a spate of emerging synthetic drugs. But in this case, the DEA is targeting two naturally occurring alkaloids in kratom, mitragynine and 7-hydroxymitragynine. Though some more controversial kratom products have been known to mimic synthetic drugs through packaging or marketing, many users who rely on the herb for treatment buy it as a powder made from pure crushed leaves. They and all kratom users would likely become felons if the DEA’s action goes through as planned.

Kratom advocates are hopeful that congressional oversight will bring more clarity to the process and expose the DEA’s war on kratom as misguided.

“We applaud Senator Hatch for taking the lead on this and hope to get some real answers as to why the DEA believes kratom should be a Schedule I controlled substance,” said Susan Ash, president and founder of the American Kratom Association, a nonprofit that works with consumers, in a statement to The Huffington Post. “We believe the facts and science needed to back up such an aggressive move simply don’t exist.”

Hatch, a well-known ally of the nutritional supplement industry, has regularly defended “natural” medicine in Congress, a cause that has attracted criticismamong those concerned about the current lack of regulation in the market. As an herbal supplement, kratom has gone largely uncontrolled at the federal level. But over the past few years, states have begun to consider age restrictions and labeling requirements, among other regulatory measures. Although these efforts have received support from some kratom groups, a federal ban would put an end to such nuanced and cooperative efforts.

The response in the Senate comes days after the House made a similar request for the DEA to delay its kratom ban. In a letter sent on Monday, a bipartisan group of more than 50 representatives called on Rosenberg to give Congress time to “engage consumers, researchers, and other stakeholders, in keeping with well-established protocol for such matters.” They also cited a number of recent or ongoing studies on kratom that suggest the herb has potential as a step-down or maintentance treatment for opioid addicts and could aid with the development of safer alternatives to prescription painkillers.

On Friday, Reps. Mark Pocan (D-Wis.) and Hank Johnson (D-Ga.) followed up with a letter to Attorney General Loretta Lynch appealing for a formal stay on the effective date of the ban in order to “allow for the appropriate notice and comment period to take place.”

The lawmakers also question the DEA’s characterization of the health risks posed by kratom, including claims that there have been “numerous deaths” linked to the herb. Most deaths associated with kratom have involved people who reportedly tested positive for numerous substances or were suffering from pre-existing medical conditions.

“Without more concrete data and analysis, it is difficult to claim Kratom represents an ‘imminent hazard,’” the lawmakers wrote.