Scientists began sampling the Hudson river this month to measure the amount of pharmaceutical pollution in the waterway.
Residue from medicine has long polluted waterways, but has only been identified as a serious problem in recent years.
Sewage treatment plants are not capable of filtering pharmaceuticals, allowing them to pass from human and animal waste to open waters.
Pharmaceutical pollution likely caused male fish in some rivers and lakes to develop female sexual characteristics in recent years.
Improper disposal of old medicines – like flushing it down the toilet – is also a problem.