Four U.S. senators told the Commissioner of the Food and Drug Administration (FDA) they disagree with the agency’s decision to approve a pure version of the painkiller hydrocodone, Newsday reports.

Senators Kirsten Gillibrand of New York, Dianne Feinstein of California, Amy Klobuchar of Minnesota and Joe Manchin of West Virginia, wrote to FDA Commissioner Margaret Hamburg that the decision “will only contribute to the rising toll of addiction and death” caused by the prescription drug epidemic.

In October, the FDA approved the first pure hydrocodone drug in the United States. The drug, Zohydro ER (extended release), was approved for patients with pain that requires daily, around-the-clock, long-term treatment that cannot be treated with other drugs. Drugs such as Vicodin contain a combination of hydrocodone and other painkillers such as acetaminophen.

In December, a panel of experts assembled by the FDA voted against recommending approval of Zohydro ER. The panel cited concerns over the potential for addiction. In the 11-2 vote against approval, the panel said that while the drug’s maker, Zogenix, had met narrow targets for safety and efficacy, the painkiller could be used by people addicted to other opioids, including oxycodone.

The agency will require postmarketing studies of Zohydro ER to evaluate the known serious risks of misuse, abuse, increased sensitivity to pain, addiction, overdose, and death associated with long-term use beyond 12 weeks.

Zohydro is designed to be released over time, and can be crushed and snorted by people seeking a strong, quick high. The opioid drug OxyContin has been reformulated to make it harder to crush or dissolve, but Zohydro does not include similar tamper-resistant features, the newspaper notes. The senators said it was irresponsible of the FDA to approve Zohydro without similar safeguards.

In an email to the newspaper, FDA spokesman Morgan Liscinsky said “abuse-deterrent formulations” are not available for some extended-release painkillers.