Thaker Hafid, 37, from Cardiff, was found dead by his wife in February after using a white powder he bought online from China.
An inquest in Cardiff heard it was found to be acetyl fentanyl, which is five times stronger than heroin.
Senior coroner Andrew Barkley recorded a narrative conclusion.
The inquest heard Mr Hafid collapsed in the study of his Fairwater home after using the drug.
Emergency services were called but Mr Hafid was pronounced dead by paramedics.
Toxicologist, Dr Simon Elliott, said of the legal high: "This is the first time we have seen this in the UK.
"It comes in a white powder and is five times stronger than heroin and 15 times stronger than morphine.
"It's designed to bypass the laws as it is legal to buy. It affects the respiratory system and switches off the brain's function to tell the body to breathe."
Dr Elliott said there was a "very strong chance" people would overdose from using the drug, but it has caused insufficient deaths to be brought to the attention of the authorities.
A post-mortem examination revealed Mr Hafid died as a result of the "toxic effects of a drug cocktail" in his blood, the inquest heard.
A father-of-three died after taking a legal high believed never to have been found in the UK before, an inquest has heard.