Before revision: Gray death is a slang term which refers to a potent mixture of synthetic opioids, for example benzimidazole opioids or fentanyl analogues, often sold on the street as a proven substance .
After revision: Gray death is a slang term which refers to a potent mixture of synthetic opioids, for example benzimidazole opioids or fentanyl analogues, often sold on the street as "heroin" .