A stable-labeled internal standard suitable for numerous GC/MS and LC/MS applications from clinical toxicology and urine drug testing to forensic analysis and isotope dilution methods. Duloxetine, marketed in the US under the trade name Cymbalta®, is a serotonin-norepinephrine reuptake inhibitor (SNRI) used to treat major depressive disorder and to alleviate pain associated with diabetic neuropathy, fibromyalgia, and osteoarthritis.