Scientists in the Republic of Ireland have found traces of cocaine on all the banknotes they tested.
The team used the latest forensic techniques that would detect even the tiniest fragments to study a batch of 45 used banknotes.
The scientists at Dublin's City University said they were "surprised by their findings".
Some of the notes had such high levels of cocaine on them that it is thought they were used to snort the drug.
Others had much lower traces and may have been cross-contaminated, perhaps in the wallets or pockets of users.