It's already known that medications that pass through the human body can get flushed into the sewer system and find their way into lakes and streams. In most cases, sewage treatment plants filter the drug residue out. But some gets through and poses a threat to fish and wildlife. What we didn't know until now was whether drugs were finding their way into our drinking water and creating a direct threat to human health.

But that's exactly what's happening. Through the experts at Edmonton's Enviro-Test Labs, we tested tap water in 10 Canadian cities to see whether the samples contained pharmaceutical drugs, such as antibiotics, prescription painkillers, and other drugs. The results were confirmed by a second lab at Trent University in Ontario.

We found drugs in the drinking water of four cities. Scientists say our test results are a wake-up call about what's happening to our water supply.

One positive test was in Brooks, Alberta, at the home of Mary-Ellen Gangl. We found traces of carbamazepine, a treatment for epilepsy. Gangl says the results are disturbing. "I'm wondering if it's affecting me," Gangl said.

Scientists want to know more too.

"We're generating a lot more questions than we have answers for," Deib Birkholz of Enviro-Test told CTV News.

We also found carbamazepine in Montreal and Hamilton. In Portage La Prairie, Man., we found gemfibrozil, a drug used to treat high cholesterol. Scientists at Environment Canada say they didn't expect the results.

"What's surprising to me is that we're able to detect it in drinking water," says the agency's Dr. Mark Servos.

All the drugs were found in extremely small amounts -- parts per trillion. That's as little as an eye drop in an Olympic-sized swimming pool. But it's not how much, it's how little we know -- such as what impact the drugs have at these levels, and whether they're harmful when mixed with other medications.

"We have no idea at this point what the long-term effects of ingestion of drugs in drinking water might be," said Chris Metcalfe, a professor of Environmental and Resource Studies at Trent University.

Health Canada started testing for drugs in the drinking water at the beginning of this year.

"There is no scientific evidence that would suggest that the drinking water is not safe at this point," Health Canada scientist Karen Proud told CTV News.

But the government is concerned. Last year, it held a conference with Environment Canada on pharmaceuticals in the water. A report from that conference, called Assessment and Management of Pharmaceuticals and Personal Care Products in the Canadian Environment, says there is "mounting evidence" that people may be at risk.

Scientists say more prescription drugs are likely to find their way into our drinking water, because prescription drugs are getting stronger and baby boomers are taking more of them as they grow older.