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updated 9/28/2010 11:16:52 AM ET 2010-09-28T15:16:52

After a recent study found traces of eight illicit drugs, including cocaine, ecstasy, and methamphetamines, in the waterways of a Spanish national park, fear started to circulate along with the headlines.

Is the environment, people have started to wonder, becoming a wasteland for discarded and partially digested medications?

As studies continue to find a growing number of pharmaceuticals and illegal drugs in a growing number of places, experts say, there are some real concerns about threats to wildlife and human health. But most substances are at levels far too low to cause problems. Many others are big question marks.

Scientists still don't know how a lot of chemicals, especially the illegal ones, might affect animals or at what level they become dangerous. Those studies just haven't been done.

"The vast majority of compounds do not pose a threat," said Paul Sibley, an aquatic toxicologist at the University of Guelph in Ontario.

"For illicit drugs, we're not quite as sure," he added. "My gut sense is that we probably don't need to be worried about this. But because studies have never been done looking at their toxicology or the responses of animals, we can't say there won't be a problem."

Whether over-the-counter or under-the-table, the most common way for drugs to escape into the environment is through the sewage system. When you pop a pill, your body breaks down some, but not all of the active chemicals in a drug. Whatever is leftover comes out in your excrement and goes down the toilet.

Most sewage treatment plants are designed to break down biological matter in substances like human waste and food scraps. But pharmaceutical chemicals often slip right through, either in wastewater that flows into streams and lakes, or in sludge that is often spread on agricultural fields.

Farm animals often consume antibiotics and other drugs, too, and their manure also helps taint agricultural run-off. From there, the chemicals end up in streams, lakes and other waterways.

Hundreds of studies have found traces of pharmaceuticals in water, Sibley said, especially downstream of treatment plants. Other studies have found these compounds in the tissues of fish and other animals. In most cases, at least in the developed world, levels are too low to have major consequences. But there are some major exceptions.

Scientists are particularly concerned about a class of pharmaceuticals known as endocrine-disruptors. Traces of estrogen from birth control pills, for example, are now known to affect animals at really tiny concentrations.

Antibiotics are another concern, because once they are unleashed in the environment, they can prompt the development of dangerously drug-resistant bacteria.

Even drugs that don't fit into those categories have been shown to cause problems in some cases, especially when levels get high enough, said Bryan Brooks, director of the Environmental Health Science program at Baylor University in Waco, Texas.

A 2004 paper in the Journal Nature, for example, documented a catastrophic vulture die-off in India. It turned out that the birds were eating the carcasses of cows that had been given a type of non-steroidal anti-inflammatory medication, similar to ibuprofen or naproxen. The drug was making the birds sick.

In a paper published earlier this year in the journal Environmental Toxicology and Chemistry, scientists reported that minnows exposed to certain antidepressants were slower to flee from predators. Another paper in the same journal issue found that tadpoles exposed to antidepressants -- at levels similar to what might show up in the environment in some places -- ate less and grew more slowly.

"We don't really have a good handle at all about how drug side effects may present risks to aquatic organisms," Brooks said. "Where the science is going now is trying to scrutinize the available data for pharmaceuticals and how they act in animals, and to prioritize which drugs may require further study."

A similar toxicological focus on illegal drugs will probably soon follow, Sibley said. The new study out of Spain looked at water in a park that is surrounded by nightclubs and malls. Many places face the same type of exposures.

"I'd be willing to say you could go to any major urban center globally, including in the U.S. and Canada, and find trace levels of these compounds, especially if you've got an active area of social life," Sibley said. "They are likely very, very ubiquitous."

© 2012 Discovery Channel

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