A platypus living in a creek or stream with waste water could be exposed to 50% of a human daily dose of antidepressants just by eating its normal diet of insects, according to new research.

A team of scientists, led by researchers at Monash University, has analysed insects and riparian spiders found in six Melbourne streams for traces of 98 different types of pharmaceuticals.

The research, published in Nature Communications on Wednesday, detected 69 different types of pharmaceuticals in insects and 66 types in spiders. It suggests the pharmaceuticals were transferred to spiders after they consumed insects.

The scientists then estimated what the potential exposure could be for the main species that feed on invertebrates in those streams: platypus and brown trout.

“There’s many studies that exist that tell us that pharmaceuticals are in the water,” said the study’s lead author, Erinn Richmond. “What we didn’t know is are these pharmaceuticals moving through aquatic food webs?

“One think I think is alarming about this study is the sheer number and types of drugs detected in these insects.

“We found 69 different pharmaceuticals.”

The streams analysed had varying levels of waste water exposure from treatment plants or other waste water sources such as leaking septic tanks or infrastructure.

The number of drug types found and their concentrations were highest in insects collected in areas downstream of waste water treatment facilities or heavily populated areas with leaking septic tanks.

But Richmond said even a site they expected to be free of contamination – Lyrebird Creek in the Dandenong Ranges national park – was found to have insects in which 41 different pharmaceutical compounds were detected.

The types of drugs detected included muscle relaxants, antihistamines, paracetamol, beta-blocking agents and small amounts of medication for Parkinson’s disease.

For some drug classes, such as antidepressants, researchers estimated that trout and platypus could be consuming as much as half the daily therapeutic dose for humans.

The scientists said pharmaceuticals making their way through food webs was not an issue specific to Australia and could occur anywhere drugs were being consumed.

They said the amount of drugs they detected was also “undoubtedly an underestimate” given they only tested for 98 compounds and more than 900 pharmaceuticals are subsidised by Australia’s pharmaceutical benefits scheme. More than 1,400 are approved by the Food and Drug Administration in the US.

But Richmond said the consequences of exposure for wildlife would require further research.

She said individuals needed to be mindful of how they were disposing of unused pharmaceutical drugs and that stricter environmental guidelines might be necessary.

“As we go on, global pharmaceutical use is increasing,” she said. “There are benefits of taking drugs for us, but the research makes it clear that these pharmaceuticals are moving through these food webs to expose insects, spiders and potentially birds, bats, fish and platypus.”