A stable-labeled internal standard suitable for quantification of fluvoxamine levels in biological samples for testing applications by mass spectrometry in clinical toxicology, forensic analysis, or urine drug testing. Fluvoxamine, sold under brand names such as Luvox® or Faverin®, is a selective serotonin reuptake inhibitor (SSRI). This SSRI antidepressant is used in the treatment of disorders ranging from major depressive and post-traumatic stress to obsessive-compulsive and anxiety.