Medicines including antibiotics and epilepsy drugs are increasingly being found in the world’s rivers at concentrations that can damage ecosystems, a study has shown.

Dutch researchers developed a model for estimating concentrations of drugs in the world’s fresh water systems to predict where they could cause the most harm to the food web.

The study, published in Environmental Research Letters, focuses on two particular drugs: antibiotic ciprofloxacin and anti-epileptic drug carbamazepine.

Between 1995 and 2015 it found that rising concentrations of the drugs and the increasing number of water tables affected meant the risks to aquatic ecosystems are 10 to 20 times higher than two decades earlier.

Carbamazepine has been linked to disrupting the development of fish eggs and shellfish digestive processes, and the study found potential risks were most pronounced in arid areas with a few major streams.

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The risks were much more widespread for ciprofloxacin, with 223 of 449 ecosystems tested showing a significant risk increase.

“The concentrations of this antibiotic can be harmful for bacteria in the water, and these bacteria in turn play an important role in various nutrient cycles,” said study author Dr Rik Oldenkamp, from Radboud University.

The environmental risks of ciprofloxacin in fresh water have increased worldwide between 1995 and 2015 (Environmental Research Letters) (R. Oldenkamp et al / Environmental Research Letters)

He added: “Antibiotics can also have a negative impact on the effectiveness of bacteria colonies used in wastewater treatment.”

More worrying still, when Dr Oldenkamp and colleagues compared their predictions to samples from four river systems they found their model was underestimating the risk.

Pharmaceutical residues can enter these fresh water systems through waste water from poorly maintained sewer systems, or from run-off over fields for drugs used in livestock.

This has become a global concern because of the risk of it accelerating the rate at which bacteria develop ways to resist treatments – rendering them ineffective when someone has a life-threatening infection.

“There’s little awareness of the role of the environment in this problem, even though it becomes increasingly clear that the environment functions as a source of resistance for various pathogens,” said Dr Oldenkamp.

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There is also a risk to the natural world if drug concentrations build up and affect animal behaviour, or disrupt bacteria performing essential decomposition and oxygenation of the soil – processes which underpin life.

Europe and other developed regions have had long-term water-quality monitoring projects; but many of the areas most vulnerable to these impacts do not, and Dr Oldenkamp said this was where their study would prove most useful.