Twenty-five senators expressed “deep disappointment” with the union rule under the law.
The AMA slammed Republicans for injecting politics into a Medicare debate.
Zohydro was approved over the advice of an FDA panel.
He is working with several other senators on a bill that would target the FDA's approval process, after the agency gave the green light in October to Zohydro, which critics say is a highly-addictive and easily-abused painkiller.
A CMS official on Tuesday said the agency lacked "statutory authority" to make such changes.
Sen. Joe Manchin (D-W.Va.) said the FDA "failed" in its duty to protect the public.
The law's "reinsurance fees" have come under fire from labor and business groups.
The move will help Democrats by preventing a wave of canceled policies.
The backlash continues to grow over what critics say is a highly addictive prescription drug.
Health groups are pushing back against congressional Republicans for efforts they say would deregulate medical technology that is crucial to the well-being of patients.
In a letter to Sen. Tom Harkin (D-Iowa), about a dozen health and public interest groups asked the Senate Health Committee chairman to fight the Preventing Regulatory Overreach To Enhance Care Technology (PROTECT) Act, which they argue could prevent the Food and Drug Administration from reviewing certain medical devices.