Description General description para-Methoxybutyryl fentanyl, also known as 4-methoxybutyryl fentanyl, is a fentanyl analog and sold on the illicit market as a designer drug. This stable-labeled internal standard is suitable for quantification of para-methoxybutyryl fentanyl levels in urine or other matrices by LC/MS or GC/MS for clinical toxicology, forensic analysis, or urine drug testing applications. Legal Information CERILLIANT is a registered trademark of Cerilliant Corporation