Prescription drugs regularly find their way into waterways where they harm fish and other aquatic life. Sewage plants could remove these substances but most balk at the prohibitive costs.
Some of the most common prescription and over-the-counter drugs found in water are antibiotics, antihistamines, pain killers, drugs used to control cholesterol, anti-depressants, and of course, hormones from birth control pills.
These medications are excreted through urine or other means (such as sweating under the shower) and then end up in sewage water plants.
But studies have shown that the concentrations of pharmaceutical trace elements in water are higher downstream from waste water plants.
Medicines in water have been blamed for a wide range of behavioral changes and mutations in fish.
In a recent test, scientists measured levels of the anti-anxiety drug oxazepam in Swedish waterways and then exposed perch in a lab to comparable levels. They found the fish were more antisocial and also more bold - behavior which could make the perch more susceptible to predators.
US scientists have conducted similar experiments and found that male minnows exposed the antidepressant Prozac (the most prescribed drug in America) ignored females and were less inclined to reproduce.
Other research has drawn a link between the female hormones in birth control pills and male fish becoming more female - their sperm count falls and they can start to produce immature female eggs.
In one Canadian study, researchers added the main ingredient found in the pill into a lake to mimic the minute concentrations found in sewage water. As a result, the fish populations suffered massive problems, and the population of minnows completely collapsed.
Cleaning up act
Sewage plants could add an extra purification step, which would partly help get rid of these drug residues. But the investment is seen as so prohibitive that most councils around Europe shy from doing it - even though the technology is relatively simple to install.
One plant that is taking the step is the Karlsruhe waste water treatment plant in Germany's southwest.
The facility lies idyllically in a patch of forest dotted with pine, beech, oaks and maples.
The rural atmosphere belies the plant's importance - it treats the water from around 700,000 homes and businesses and is the second biggest in the state of Baden Württemberg.
Michael Steinert, the facility's deputy head, gestures to the waste water intake tank, where feces, toilet paper, cigarette butts, orange peel and other muck float in the turgid, brown water.
From here, the sewage will spend 12 hours flowing through a variety of purification steps before being released into a canal, where it flows into a small stream and then the Rhine River.
First up, Steinert says, is a mechanical process, which involves feeding the water through grates which trap the larger debris. It then goes through a grit collector and a grease trap.
Steinert then strides over a metal walkway to covered tanks filled with bacteria and other micro-organisms.
In this biological process, the bacteria feast on the carbon and nitrogen compounds and phosphorus in human waste, removing a large part of them from the sewage.
But that's not everything.
"What we can't yet remove are pharmaceutical trace elements" says Steinert. "The residues aren't large enough be filtered out in the mechanical part of the treatment process. And many pharmaceutical products are explicitly designed so that they can't be broken down by bacteria."
The Karlsruhe water waste treatment plant is spending 50 million euros to build an additional purification basin where a substance called powdered activated carbon will be added to the waste water.
Activated carbon is charcoal that has been treated to open up millions of tiny pores between the carbon atoms. This makes the material extremely absorbent.
"These substances have proved to have a very high affinity with activated carbon which absorbs eighty, ninety, even one hundred percent of the micropollutants," says Martin Maurer, the head of the Karlsruhe facility.
The carbon becomes part of the sewage sludge, which is removed, partly dried and incinerated.
"That way, the carbon is used as an energy source and the trace elements are completely destroyed," Maurer says.
Not everyone, though, sees the point of spending money to remove such minuscule trace elements from the water.
"An aspirin has 500 milligrams, so we would have to drink several million liters of water to get rid of a headache," laughs Maurer. "And because we are talking about nanoparticles, some say the extra processing is throwing money down the drain."
However, an increasing number of scientists believe the total sum of the pharmaceutical products - the drug cocktail - will become an increasing threat for aquatic life, especially as our aging populations take more and more medication.
"This means the additional purification process is absolutely essential," Maurer says, "for our waterways, for the Rhine and for the North Sea."