"We know there are good drugs and bad drugs in Canada, but we can't tell you which ones are which," says William Hubbard, the FDA's associate commissioner for policy and planning. "We don't have the regulatory reach."

"If you walk into a Canadian pharmacy, you're going to get medication that is safe," says Carmen Catizone, executive director of the National Association of Boards of Pharmacy (NABP). "If you order from the Internet, you have no way of knowing that's going to occur."

Canadian trade figures indicate that the concern may be warranted. A Prudential Financial research report last fall revealed that pharmaceutical exports to Canada were up 300% from Bulgaria, 196% from Pakistan, 171% from Argentina, 114% from South Africa and 101% from Singapore. Although those numbers are not concrete evidence of so-called transshipping, it suggests that it may be happening, says Diane Duston, the analyst who co-wrote the report.

"We began directly notifying a group of Internet pharmacies last August that they would no longer get Pfizer products," says Pfizer spokesman Jack Cox, who adds that Canadian pharmacies are violating the terms of their contracts when they export Pfizer drugs to the United States. (Popular Pfizer products include Celebrex for arthritis, Lipitor for high cholesterol, Norvasc for high blood pressure and Zoloft for depression.)

The crackdown has created shortages, and pharmacies that export to the United States are scrambling to fill orders. the pharmacy-to-pharmacy trade that keeps Internet operations in stock is now threatening to create shortages for Canadians.

I don't know if the risk of getting inferior drugs is 1% or 10% or 90%," Hubbard says, "but it's a risk, and it's not fair that senior citizens have to take it."