Police have issued warnings about a legal high called Benzo Fury following the death of a 19-year-old man at the RockNess Festival.
The teenager, from Edinburgh, was taken ill in the main arena late on Saturday night.
Northern Constabulary police officers said they were investigating the possibility he had taken drugs.
Later police issued photographs of a substance called Benzo Fury, saying they believed it to be "in circulation" and believed it "may be dangerous".
A statement said: "Police and health professionals have issued a warning to people not to take non-prescribed medicines.
"Advice is that the term 'legal' does not mean 'safe'.
"Advice to anyone in possession of this drug is not to consume it."
Police said the 19-year-old, who has not been named, was taken to the hospital tent and stabilised before being transferred to Raigmore Hospital in Inverness.
He failed to respond to treatment and died in the early hours of Sunday morning.
A statement from the Northern Constabulary, said: "Early indications are that the man may have consumed drugs and this is one of the lines of enquiry at this stage.
"A post mortem will be carried out to determine the cause of death."
The teenager was visiting the festival from Portobello in Edinburgh.
Meanwhile, police have named two men who were killed on Friday in a crash involving a bus taking people from Edinburgh to the festival.
A van driver and his passenger died of their injuries following a collision with a bus on the A9 at Ralia, in the Highlands.
They are Mark McFarlane, 38, and Barry Murray, 28, both painters and decorators from Glasgow.
The bus driver was airlifted to the Southern General Hospital in Glasgow with serious injuries after she became trapped in the vehicle.
Fourteen passengers were taken to Raigmore Hospital for treatment to a range of injuries, none of which are life-threatening.
RockNess draws about 35,000 people over the three days it is held and it takes place on the banks of Loch Ness.
Headline acts this year include Ed Sheeran, Mumford & Sons and Biffy Clyro.