WASHINGTON – Congressional investigators found that prescription drugs obtained from Canadian Web sites pose fewer risks than medications purchased from online pharmacies elsewhere.

In some instances, Canadian pharmacies had stricter standards than those in the United States, according to the report by the General Accounting Office.

The report was being released in conjunction with a Senate Governmental Affairs subcommittee hearing Thursday where prominent opponents of imported drugs were to testify.

Lawmakers who advocate drug imports from Canada and elsewhere are trying to force a Senate vote to legalize the practice. The Food and Drug Administration has said it cannot guarantee the safety of the foreign products.

Older Americans have flocked to Canada for prescription medications as drug prices in the United States have soared and fixed incomes have not kept up, advocates say.

Several bills would strengthen federal regulation of Internet pharmacies and inspections of pharmaceutical manufacturing plants abroad.

GAO investigators purchased drugs from Internet pharmacies in the United States, Canada, Mexico, Spain, Thailand and seven other countries. All 18 Canadian sites required consumers to supply a physician-written prescription before filling orders. That was the case for five of 29 U.S. pharmacies; no other foreign pharmacies did.

Prescriptions filled in Canada and the United States came with labels from the dispensing pharmacy and generally included patient instructions and warnings, said the report by the investigative arm of Congress.

The biggest problem investigators noted was that drugs shipped from Canada did not have FDA approval for use in the United States for reasons such as production in unapproved plants or carrying different labels.

But the medicines had a comparable chemical composition to approved pharmaceuticals, the report said.