The fake products do not contain the key ingredient in Avastin, which is used to treat colon, lung, kidney and brain cancer.
Roche says the counterfeits do not have "Genentech" printed on the packaging.
Additionally, legitimate Avastin contains a six-digit lot number with no letters. All the packaging text should be in English.
The company believes drugs labeled with the following lot numbers may be fake: B86017, B6011 and B6010.
Doctors who suspect they've received fake products should contact the Food and Drug Administration.
Roche and the FDA are working together to track down the counterfeit vials and analyze their contents.
The FDA said on Tuesday that it has contacted 19 medical practices that may have purchased unapproved drugs, including counterfeit Avastin, from a company called Quality Specialty Products. The foreign supply company may also do business as Montana Health Care Solutions, the agency said.
The Associated Press contributed to this report.
