Traces of medicine are not only hard to remove from wastewater, some have even higher levels after the water is cleaned, a US study has found.

Scientists in Milwaukee took samples from a sewage plant and found that after treatment there were 47 different pharmaceuticals still in the water.

They also noticed something very odd – there were more of two of these drugs than to start with. Carbamazepine, an anti-epileptic, increased by 80 per cent and ofloxacin, an antibiotic, increased by 120 per cent.

Lead author of the University of Wisconsin-Milwaukee study Benjamin Blair told Environmental Health News that he believed the microbes used to clean the water were causing the drugs to multiply.

“Microbes seem to be making pharmaceuticals out of what used to be pharmaceuticals,” he said, explaining that when people take medicine, it passes into the water broken down into different bits. Microbes could well, according to Dr Blair, be piecing them back together in the plant.

Canadian researchers in Ontario found similar results when they tested water in a Peterborough plant.

It was not clear to the team why only two drugs of the 48 increased in the cleaning process.

Though not thought to be harmful to humans, the US Geological Survey (USGS) found that levels of pharmaceuticals did have an impact on wildlife - specifically on the reproductive habits of fish.

Scientists at the USGS monitored water flowing into estuaries near a New York filteration system where fish breeding levels have been declining. There were high levels of medicines found in the water.

The USGS found out that traces can stay in irrigated soil for months – meaning there’s a chance they can transfer into plants.