Concrete, malaria medication and insecticide were all found in drugs brought to an on-site laboratory which allows festival goers to test their stashes without fear of arrest.
A charity rolled out the facility at the Kendal Calling festival in Cumbria to help users detect the potentially fatal ingredients in a bid to save lives.
They promised that users who turned up get drugs including cocaine and ecstasy tested for the any toxic agents will not be arrested or face any criminal charges.
The Loop charity said it was offering the “pragmatic, harm reduction initiative” to help curb drug deaths for fans who want to enjoy the festival while under the influence.
Its director Professor Fiona Measham said: "We accept that some people will get drugs on site and some people will be planning to take them so what we're doing is trying to address any potential health problems.
"This is a focus on public health rather than on criminal justice."
But furious critics hit out that the move “normalises” drug taking and could lead to an increase in people taking the illegal substances.
It comes after Christian Pay, 18, died after taking an ecstasy pill at the Kendal Calling festival in 2015 – which police said was part of a batch of the drug that left several people hospitalised.
About one in five users ask the charity to dump their substances after they have been tested, Prof Measham said.
But critics argue that it could lead to an increase in drug use.
David Raynes of the National Drug Prevention Alliance told Sky News: “This normalises drug taking. Some people go to festivals for the first time and take drugs for the first time.
“The drugs they take will not be drugs that have been tested because during the testing process the drugs get destroyed so there will be other drugs available to them.
“Testing doesn't make the drugs that people might take at a festival safe.”
One man, who did not want to be identified, brought an ecstasy tablet to Kendal Calling festival for examination. Results showed that it was pure.
He said: “I read some bad reports about the pills as well as some good ones, so I decided it would be best for me that I have the reassurance in my mind to know that I've come here, have it tested and know for sure.
“It just gives me peace of mind to know that what I'm taking is safe instead of just taking anything.”
The testing comes after a number of drug-related deaths at UK festivals in recent years.
Six festivals, including Reading and Leeds, are to allow on-site drug testing this year in an attempt to highlight potentially lethal batches of drugs.
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