Chemists have found that United States currency contains the largest trace amounts of cocaine in the entire world--between 2.9 and 28.8 micrograms.  Spain had the largest trace amounts of any European country.

This does not necessarily mean that every paper bill is being used by U.S. citizens to get their fix--cocaine can easily make its way to bills from ATMs, vending machines, etc.  It does however mean that Americans are probably doing a sh*tload of cocaine.