The first package was intercepted Nov. 26 at an air mail facility near San Francisco International Airport, said Roxanne Hercules, a spokeswoman for U.S. Customs and Border Protection.
Since then, agents have seized 51 separate packages, each containing up to 50 counterfeit capsules labeled generic Tamiflu.
The fake drugs had none of Tamiflu's active ingredients, and officials were running tests to determine what the capsules did contain. Initial tests indicated some vitamin C in the capsules, said David Elder, director of the Food and Drug Administration Office of Enforcement.
Information on the packages was written in Chinese, but it is unclear where the drugs originated, Elder said.
They were sent by Asian suppliers to individuals who placed orders over the Internet, Hercules said. She said none of the shipments intercepted so far was bound for doctors or hospitals.
Agents became suspicious because Tamiflu is produced by Swiss pharmaceutical manufacturer Roche, and there is no generic version available.
"What we're trying to do is alert the American public that they shouldn't be buying this product because we may never be able to track down the manufacturers," Elder said Sunday. "We've anticipated the likelihood of counterfeits from the very beginning. People are trying to profit on the heightened concerns of the American public."
The H5N1 strain of the bird flu virus has ravaged poultry stocks in Asia and killed at least 71 people since 2003. Tamiflu is one of two drugs found to be effective against it.
Fears of a bird flu pandemic have increased as the virus has spread into poultry in Europe. While bird flu has not yet appeared in the United States, or been found to transmit from person to person, officials worry it could mutate and, because humans have no immunity to it, become contagious globally.
President Bush has requested $7.1 billion in emergency flu preparedness funds.
Roche declined to comment Sunday. Its Web site says the company "does not advocate the purchase of Tamiflu via the Internet. Patients should always gain a diagnosis from a health care professional before buying Tamiflu and ensure they obtain it from a reliable source."
The FDA will seek criminal charges if it finds any U.S. businesses were involved in the fake drug shipments, Elder said.