Environmentally damaging levels of drugs have been found in the river running through the Glastonbury Festival site.
Scientists have warned there are "dangerous" levels of MDMA and cocaine in the Whitelake River in Somerset.
They suspect public urination has caused the increase and urged festival goers to use the toilets provided.
Researchers fear it could derail the conservation efforts of rare European eels in the area.
Measurements both upstream and downstream of the site were taken before, during and after the festival in 2019.
The study found MDMA concentrations quadrupled the week after the festival, suggesting long-term release from the site.
A spokesman for Glastonbury Festival said the Environment Agency had not raised any concerns with them on the matter following the 2019 event.
He said: "Protecting our local streams and wildlife is of paramount importance to us at Glastonbury Festival and we have a thorough and successful waterways sampling regime in place during each festival, as agreed with the Environment Agency.
"We are aware that the biggest threat to our waterways - and the wildlife for which they provide a habitat - comes from festivalgoers urinating on the land."
It does not condone the use of illegal drugs and continues to "successfully strongly discourage" public urination, he added.
"We are keen to see full details of this new research, and would be very happy to work with the researchers," the spokesman said.
'Public urination happens'
Dr Christian Dunn, from Bangor University, said: "Our main concern is the environmental impact. This study identifies that drugs are being released at levels high enough to disrupt the lifecycle of the European eel.
"We [also] need to raise awareness around drug and pharmaceutical waste - it is a hidden, worryingly-understudied yet potentially devastating pollutant."
Dan Aberg, a Masters student in the School of Natural Sciences at the university, added: "Illicit drug contamination from public urination happens at every music festival.
"Unfortunately, Glastonbury Festival's close proximity to a river results in any drugs released by festival attendees having little time to degrade in the soil before entering the fragile freshwater ecosystem."
Further studies into possible treatment via environmentally friendly methods in order to minimise the release and impact of illicit drugs from festivals have been suggested.