Why are some tricyclic anti-depressants are good for anxiety disorders?

Norepinephrine: Tricyclics typically work on more receptors than the ssris, which target serotonin. Some people find that their particular depression or anxiety symptoms respond better to medications that work on norepinephrine or acetyocholine.

We don't know: Nearly all antidepressants turn out also to be good for treating anxiety too. We still do not understand exactly how they produce their effects on either depression or anxiety. Tricyclic antidepressants were among the first to be used widely; they are less commonly used now because they have more side effects than new antidepressants but can stil be very useful.