By Shelia Kaplan l The New York Times

The Food and Drug Administration on Thursday said a major Canadian drug distributor was selling unapproved and mislabeled medicines to unsuspecting Americans looking to save money on prescriptions, and warned it to stop.

The company, CanaRx, sells many common prescription medicines at a lower cost to hundreds of public and private employer programs in the United States. Many of its buyers are city and county governments seeking to save money, among them: Albany, N.Y. and Duluth, Minn. CanaRx says the medications it sells are high-quality, from Canada, Australia and Britain, but the F.D.A. said this is not always the case.

“Operations like CanaRx use their names to imply that patients are receiving medicines approved in Canada, when it’s likely that some of those drugs are from other countries with lax standards,” Dr. Scott Gottlieb, the F.D.A. commissioner, said in an interview. “Such operations take advantage of unsuspecting Americans, by purporting to distribute safe and effective imported drugs, at least some of which are instead expired, mislabeled, subject to recalls or potentially counterfeit.”

A statement posted on the agency’s website urged consumers not to use any medicines obtained from CanaRx.

Joseph Morris, general counsel for CanaRx, said in an interview: “Every prescription that is dispensed through a CanaRx program is dispensed directly to the patient from a licensed, regulated, brick-and-mortar pharmacy in Canada, Britain, or Australia, and the patient can be sure that medicine she receives is the medicine that her doctor ordered.”

The F.D.A. did not present any evidence that the CanaRx drugs came from countries other than those it advertised, but said its investigation was ongoing.

The F.D.A.’s action comes at a time of renewed political attention to drug importation as an alternative for Americans facing expensive prescription medicines at home. Senator Bernie Sanders, the Vermont Independent running for president, and House Democrats Elijah Cummings of Maryland and Ro Khanna of California, have introduced bills that would allow patients, pharmacists and wholesalers to import medicine from Canada and other countries and would peg the price of prescription drugs in the United States to the median price in Canada, Britain, France, Germany and Japan.

Some states are also moving to take action. In Florida, Governor Ron DeSantis recently proposed importing Canadian drugs to his state to save money. Vermont last year enacted a prescription-drug importation law, although it has not yet gone into effect.

Federal law bars importing unapproved prescription drugs into the United States, although millions of Americans do so. The Department of Health and Human Services has the power to approve specific programs permitting drugs from Canada to enter the country, but so far has not done so, the agency said.

In answer to a question at a congressional hearing on Wednesday, Dr. Gottlieb said he was certain that prescriptions sold by Canadian pharmacies were safe, but not that online websites claiming to have bought their drugs in Canada really had done so.

Dr. Gottlieb said he was especially concerned about CanaRx’s sale of drugs with special safety requirements because they were high-risk and needed to be carefully managed to protect vulnerable patients. Among the drugs the agency’s warning letter cited were Tracleer, a prescription medicine for patients with certain types of pulmonary arterial hypertension — high blood pressure in the vessels of the lungs — and CellCept, prescribed for people who have received a kidney, heart or liver transplant.

“CanaRx is causing important safety measures that are put in place for the FDA-approved versions of these drugs to be bypassed,” the warning letter said.

Mr. Morris, the CanaRx lawyer, said the company would cooperate with the F.D.A. He added that he thought the agency might have confused some websites that advertised or sold imported drugs with CanaRx. “Many of the websites listed in the F.D.A. letter belong to other entities completely unconnected to CanaRx,” he said.

In an email to The New York Times, Mr. Morris wrote that 123 of the websites listed were active CanaRx websites; 27 were CanaRx programs that were terminated, and 15 were not affiliated with the company. An F.D.A. spokesman said the agency stood by its findings.

CanaRx has become popular over the past 15 years with many employers in the United States. Mr. Morris said the company serves as a broker between the companies’ employees and pharmacies and physicians in Canada, Australia or Britain. The consumer sends a prescription to CanaRx, which then finds a foreign doctor to reissue it, and have it filled locally, he said. The drug is then sent to the American consumer.

“The cost savings are often very great, which benefits the individual consumer and their plan sponsors,” Mr. Morris said.

The warning letter requested that CanaRx immediately stop selling unapproved new drugs from foreign sources to American buyers. It noted that CanaRx’s disclaimer told consumers that their medicine may have appeared to be different in size, shape or color than what the consumer had bought in the United States.

“Having this disclaimer in each invoice demonstrates that CanaRx has designed its business to operate in a manner that substitutes the F.D.A.-approved drugs prescribed by the U.S. health care provider with unapproved drugs,” the F.D.A. wrote.