Police Scotland made the plea ahead of TRNSMT kicking off in Glasgow on Friday.
The force issued the advice after officers recovered the substance - previously seen south of the border - in Paisley.
It looks like a paper stamp but contains an “extremely harmful” Class A drug.
Chief Superintendent John McKenzie said: “As people will be aware, officers in Paisley recently recovered a Class A drug that’s previously been seen in England, which looks like a paper stamp but is impregnated with an extremely harmful drug.
“There are often different substances being sold in a similar fashion which make it impossible to visually identify what drug or strength of drug is being sold.
“People have been hospitalised after taking it, so our advice is if you’re offered this, or any other illegal drugs, don’t take the risk.”
Officers will be at the music festival to assist, while revellers have been asked to stay safe and remain vigilant.
People attending TRNSMT may be searched as a condition of entry and certain items will not be permitted.
There has been no specific intelligence to suggest any threat at the event, Chief Superintendent McKenzie said, adding: “We want to help create a safe and friendly atmosphere.
“So as well as people taking these simple steps to stay safe, they will see an increased security presence at festivals, starting with TRNSMT in Glasgow on Friday.
“It is important to stress there is no specific intelligence to suggest any threat to this festival, but given the current threat level of “severe” our officers will be on site throughout the weekend.
“Our advice remains the same - be alert but not alarmed and enjoy the festivals.”
Organisers of the event are urging attendees to plan their journeys ahead.
Anyone making the journey by coach has been told they must book in advance while ScotRail has already announced it will be adding capacity to its services in anticipation of increased pressure.
A number of road closures will be in place for the duration of the festival.
Blue Badge parking will be on the East Carriageway between Binnie Place and Arcadia Street. Access to the parking area will be from Arcadia Street.
The festival is taking place between Friday and Sunday as well as on Friday, July 6 and Sunday, July 8.
Travel options to the TRNSMT site can be found at www.TRNSMTfest.com/travel.