APADIN (Doxepin) is a tricyclic antidepressant with anxiolytic (anti-anxiety) properties.  As Doxepin Hydrochloride, it is the active ingredient in Zonalon and Xepin, cream preparations for dermatological itch.  But Doxepin is primarily used as a treatment for insomnia.

Doxepin primarily inhibit the reabsorption of Serotonin and Norepinephrine to other nerve cells, therefore creating higher levels in the brain. To a lesser extent, Doxepin also affects Dopamine.  It is the blocking of cells receptors that accounts for the many side effects.

Norepinephrine is both a hormone and a neurotransmitter. As a hormone (secreted by the adrenal gland), it works with epinephrine / adrenaline to give the body sudden energy in times of stress. This is known as the "fight or flight" response. As a neurotransmitter, it passes nerve impulses from one nerve cell to another. Norepinephrine plays a role in elevating mood, memory and anxiety, and reactions to stress.

Serotonin acts as both a chemical messenger that transmits nerve signals between nerve cells and causes the blood vessels to narrow.  Serotonin makes blood clots form and is a muscle as well as a vasoconstrictor, but it also plays an important role in sleep, appetite, memory, aggression, sexual behavior, cardiovascular activity, respiratory activity, motor output, neuroendocrine and sensory function, and perception. According to Dr. Ann Blake Tracy, an increase in Serotonin produces rushes of insulin that drops blood sugar levels and can create a chemically induces hypoglycemia (low blood sugar). Additionally, too much Serotonin damages blood vessels, particularly in the lungs and may adversely affect heart valves. This is because Serotonin is a powerful vasoconstrictor (narrows the blood vessels).

Like other Tricyclics, Doxepin also blocks sodium channels, the protein responsible for excitability of neurons.  This may account for the analgesic (painkilling) action. But Doxepin also exerts an anesthetic action, which can cause a loss of pain sensation as well as muscle paralysis.