Scientists have found drugs including cocaine and ketamine in shrimp collected from rivers in Suffolk.
A study by researchers at King's College London and the University of Suffolk also discovered traces of banned pesticides and pharmaceuticals in samples taken from the county's waterways.
The team collected samples from five catchment areas and 15 sites - and found cocaine in all of them.
In a study published in the journal Environmental International, the researchers said medicines and drugs end up in rivers after use and could cause pose a risk to wildlife.
Leon Baron, from King's College London, said the findings were unexpected for a county like Suffolk.
Concentrations of drugs in the shrimp were low and the scientists said it was not clear whether this was a problem specific to Suffolk or if it could be a more widespread occurrence across the UK.
They called for further research.