Three people suffered "serious side-effects" after taking a tablet they believed to be a class A drug at the V Festival in Weston Park, Staffordshire.
Staffordshire Police said: "Emergency teams have treated people this evening 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."
The tablets were known as "Blue Ghost" and police said they had already seized a small number of them.
It is believed the trio reported stomach problems, though none required hospital treatment.
A festival spokesman 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."