4-ANPP, also known as 4-anilino-N-phenethylpiperidine (4-ANPP), 4-aminophenyl-1-phenethylpiperidine, or despropionyl fentanyl,[3] is a direct precursor to fentanyl and acetylfentanyl. It is commonly found as a contaminant in samples of drugs containing fentanyl, which may include samples represented by the supplier as heroin or other opioids.[4] It is not psychoactive and is present only as a result of improper chemical purification.