Pharmaceutical company Roche issues warning about counterfeit cancer-fighting drug Avastin.
"The investigation is ongoing and we are assisting the authorities" in the United States, a spokeswoman for the Swiss firm said.
Genentech, Roche's US subsidiary, warned in a statement, "The counterfeit product is not safe or effective and should not be used.
"Chemical analyses of the counterfeit vials tested to date have confirmed the product does not contain the active ingredients for Avastin."
Some of the counterfeit US lots are believed to be labeled as Avastin 400 mg/16 ml and have the serial numbers B86017, B6011 and B6010.
Genentech said the suspect batches differ from the official product, which is certified by the US Drug Administration (FDA).
Genuine Avastin boxes are normally marked with six digit serial numbers and have no letters, stated the company.
Avastin is used to treat colorectal, lung, kidney, breast and ovarian cancer and is among Roche's top-selling drugs with a turnover of 5.3 billion Swiss francs ($5.8 billion, 4.4 billion euros) in 2011.