Dangerous drugs have left several festival-goers needing treatment from paramedics.
Drug advisory service The Loop has issued a warning about a so-called blue "Anonymous" pill has been causing problems at the Kendal Calling festival.
Police have seized a "significant amount" of the illegal drugs after they were made aware of their presence by paramedics and The Loop.
It is believed that due to the amount they are being sold in the festival, which is held at Lowther Deer Park, near Penrith, this weekend.
Fiona Measham, The Loop's director, said: "We tested these pills and they turned out to be pentylone which is a long-lasting stimulant which leads to increased heart rate and can lead to paranoia and high blood pressure."
The pills are believed to have been sold as ecstasy.
"Ourselves and the police have been putting warnings out so say don't take these," Prof Measham added.
Thousands are attending the festival.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here