Roche was contacted about the bogus Avastin by a health authority outside the United States and was informed that the counterfeit drug in the United States came from another country, the company said but declined to divulge which country.

"We are working with the FDA and law enforcement to aid their evaluations, determine the source of the counterfeit drug, and prevent its further distribution," Roche and Genentech said in a statement. "The counterfeit product is not safe or effective and should not be used."

The U.S. Food and Drug Administration is taking the lead on the evaluation, a Genetech spokeswoman said.

Genentech said there is an ongoing investigation by national health authorities but could provide no further information at this time.

The FDA said it has sent letters to 19 medical practices informing them about counterfeit 400mg/16ml doses of Avastin.

The company does not yet know just how much counterfeit Avastin is out in the market. Avastin is given intravenously.

But there are several obvious differences in packaging and label that should allow doctors to easily spot the bogus drug.

Among them genuine Avastin, known chemically as bevacizumab, has Genentech on the label, which is all in English. The counterfeit says Roche and the label is in French.

Lot numbers of actual Avastin are comprised of six digits with no letters, while the counterfeit lot number begins with a letter. And the counterfeit bottles of Avastin are missing information on the label, such as "for intravenous use."

(Reporting By Bill Berkrot)

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