Description General description This stable-labeled internal standard is suitable for quantitation of U-50488 levels in urine, serum, or plasma by LC/MS or GC/MS for clinical toxicology, forensic analysis, or isotope dilution methods. U-50488 is a potent opioid agonist, not structurally related to fentanyl. U-50488 belongs to the class of novel synthetic opioids (NSOs) created to circumvent drug control laws with no recognized therapeutic use. NSOs are clandestinely manufactured and sold on conventional or dark web.