Three people required medical care at the V Festival after taking a pill they believed to be ecstasy on Saturday (August 16).
Staffordshire Law enforcement officials confirmed that the drug - nicknamed 'Blue Ghost' - caused "serious side effects".
It was also confirmed that a batch of the pills was seized.
Staffordshire Police told ITV News: "Emergency teams have treated people [on Saturday] who took a blue tablet they believed to be ecstasy at V Festival in Weston Park.
"All had serious side-effects after taking the tablets and needed urgent medical treatment."
A festival spokesman also said: "We take these reports seriously and will do everything in our power to support the police in their investigations to ensure the festival continues to be a safe environment for everyone."
Justin Timberlake had the headlining slot at the V Festival on Saturday (August 16).
