That's an important question; it's often difficult. You are probably aware that we have been experiencing a problem with the drug called heparin in the United States. Heparin is a drug that's been available for decades. It's one of the oldest drugs we have, and its safety profile is relatively well known because it has been around so long and has been so widely used. Last November and December there were clusters of outbreaks in dialysis centres around the United States. Multiple patients were getting allergic reactions. Although in rare cases patients receiving heparin can get an allergic reaction, these clusters were particularly unusual.
Our Centers for Disease Control and Prevention, another government health agency, did an outbreak investigation and determined that it was the heparin from one particular manufacturer that was responsible for the outbreak. The heparin was put through very complicated testing and it turned out to be contaminated, even though it came from the manufacturer. In these cases, we're not really dealing with a counterfeit product but with a contaminant. Since this was a relatively widespread problem, we were able to identify both the cause, which was this particular manufacturer's heparin, and the contaminant.
It may not always be easy to determine that an adverse reaction is caused by counterfeit drug rather than a properly manufactured one. If the drug safety profile is well known and the drug has been around a long time and you start seeing adverse reactions that don't fit the pattern, you might suspect the counterfeit, but it would be difficult. If you don't have the actual medication to test, it would be even more difficult.