Traces of almost 30 different pharmaceutical drugs, including antidepressants, antibiotics and medicines to treat epilepsy and diabetes, have been found in rivers in York
Scientists said they found 29 different drug compounds in the River Ouse and River Foss, including some from drugs not usually available in the UK, thought to have been ingested by American and Chinese tourists.
The York University team stressed that although the levels were extremely low - drinking two litres of river water would give you about a millionth of a patient's daily dose of one drug - there were concerns over the long-term implications.
They said that some drugs showed levels higher than previously observed across parts of Europe and Asia.
The research, led by Emily Burns in the university's Department of Chemistry and overseen by Professor Alistair Boxall from the Environment Department, looked at samples from 11 sites in the city over a 12-month period.
Professor Boxall said: "There isn't any evidence for impact on human health from pharmaceuticals in rivers, but it definitely deserves more investigation.
"If you compare the levels found in the study with what a patient takes then the degree of exposure is very low. For example, we saw concentrations of metformin - a drug used to treat diabetes - at around 500 nanograms per litre.
"If you drank two litres of this water, you would get about a millionth of a patient's daily dose of the drug."
Professor Boxall added: "However, it is important to realise that these drugs are being emitted continuously into the environment and that we will be exposed to them across our lifetime. There is therefore a concern that some may be causing harm.
"It is a really complex issue to tackle and we don't really have the methods to understand whether long-term exposure to low levels of pharmaceuticals matters or not."
Professor Boxall said there were also questions over the impact of the drug traces on the ecosystem.
The research, published in Water Research and funded by the EU's Seventh Framework Programme, showed seasonal spikes, with higher levels of antihistamines in the summer and higher levels of drugs associated with cold and flu symptoms in the winter.
The team said the study has laid the foundations for a new global project led by York University, looking at water samples taken from 60 rivers around the world.
They said the results of this should be known by the end of the year.