River systems around world are coursing with over-the-counter and prescription drug waste harmful to the environment, researchers said Tuesday.

On current trends, the amount of pharmaceutical effluence leaching into waterways could increase by two-thirds before mid-century, they told a major science conference in Vienna.

"A large part of the freshwater ecosystems is potentially endangered by the high concentration of pharmaceuticals," said Francesco Bregoli, a researcher at the Delft Institute for Water Education in the Netherlands, and leader of an international team that developed a method for tracking drug pollution "hotspots".

A large number of drugs found in the environment -- analgesics, antibiotics, anti-platelet agents, hormones, psychiatric drugs, anti-histamines -- have been detected in nature at levels dangerous for wildlife.

Endocrine disruptors, for examples, have notoriously induced sex changes in fish and amphibians.

Bregoli and his team used a common anti-inflammation drug, diclofenac, as a proxy, or stand-in, to estimate the presence and likely spread of other medications throughout freshwater ecosystems.

Both the European Union and the US Environmental Protection Agency have identified the drug as an environmental threat.

Veterinary use of diclofenac, for example, has driven a sub-species of vultures on the Indian subcontinent to the brink of extinction.