Counterfeit versions of Roche and Genentech's multi-billion-dollar cancer drug Avastin have been detected in the US, prompting concerns over patient safety.

The US Food and Drug Administration said it has sent out letters to 19 medical practices - which have all purchased unapproved cancer medicines from Quality Specialty Products (QSP), a foreign supplier that may also be known as Montana Health Care Solutions - warning healthcare professionals about a fake version of Avastin 400mg/16mL. 

The counterfeit form is labeled as Avastin, manufactured by Roche, and does not contain the active ingredient bevacizumab, which may have left patients without their therapy. The only FDA OK'd version of Avastin is marketed by Genentech, and does not include the Roche logo on the packaging or vials. 

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

It is still unknown how many vials of fake Avastin have infiltrated the US supply chain, and no details have been released on where they might originate from.

The counterfeiting and contamination of medicines has become a huge issue for countries like the US in which many pharmaceutical companies have shifted production to foreign shores. 

Around 80% of active ingredients used in US prescription drugs now come other countries, media reports note, leaving the supply chain much more vulnerable and patients at greater risk.