However, seek immediate medical attention if you notice any symptoms of a serious allergic reaction, including: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. Doxepin belongs to the class of tricyclic antidepressants (TCAs). Drowsiness, dizziness, dry mouth, blurred vision, constipation, or trouble urinating may occur. Doxepin may cause a condition that affects the heart rhythm (QT prolongation). If you think you have used too much of this medicine seek emergency medical attention right away.