A study assessed the metabolism of a single 75 mg oral dose of doxepin in healthy volunteers with genetic polymorphisms in CYP2D6, CYP2C9, and CYP2C19 enzymes. In CYP2D6 extensive, intermediate, and poor metabolizers, the mean clearance rates of ("E")-doxepin were 406, 247, and 127 L/hour, respectively (~3-fold difference between extensive and poor). In addition, the bioavailability of ("E")-doxepin was about 2-fold lower in extensive relative to poor CYP2D6 metabolizers, indicating a significant role of CYP2D6 in the first-pass metabolism of ("E")-doxepin. The clearance of ("E")-doxepin in CYP2C9 slow metabolizers was also significantly reduced at 238 L/hour. CYP2C19 was involved in the metabolism of ("Z")-doxepin, with clearance rates of 191 L/hour in CYP2C19 extensive metabolizers and 73 L/hour in poor metabolizers (~2.5-fold difference). Area-under-the-curve (0–48 hour) levels of nordoxepin were dependent on the genotype of CYP2D6 with median values of 1.28, 1.35, and 5.28 nM•L/hour in CYP2D6 extensive, intermediate, and poor metabolizers, respectively (~4-fold difference between extensive and poor). Taken together, doxepin metabolism appears to be highly stereoselective, and CYP2D6 genotype has a major influence on the pharmacokinetics of ("E")-doxepin. Moreover, CYP2D6 poor metabolizers, as well as patients taking potent CYP2D6 inhibitors (which can potentially convert a CYP2D6 extensive metabolizer into a poor metabolizer), may be at an increased risk for adverse effects of doxepin due to their slower clearance of the drug.