Police in Essex have issued a warning to anyone considering taking legal highs along to this weekend's V Festival at Hylands Park in Chelmsford.
Officers have vowed to have a zero-tolerance approach and have warned festival-goers they will face arrest if found in possession of any substance.
While it is claimed legals highs do not contain any illegal substances, police said no-one could be sure what was in them.
Chief Inspector Jon Hayter said that meant people taking them were putting themselves at serious risk.
But it also left officers with no choice but to arrest anyone found in possession of a substance in case it did include illegal drugs.
Last year, Jonathan Graham, who was 33, died after taking what was thought to have been a so-called "legal high" at the Brownstock music festival in Essex.