Why are antidepressants used to treat migraine?

They are effective. Some antidepressants act on certain Dopamine or serotonin receptors. It is thought that the symptoms of migraines can be reduced by interfering with the above receptors as an effective treatment. Antidepressants have many uses and just because you use one does not mean you have depression or psychiatric disorders. Take only asdirected and mention if you are taking any other med or supplement.
Usually Tricyclics. Tricyclic antidepressants, particularly amitriptyline, can be used as a treatment to prevent migraines in some people. A person does not have to have depression in addition to migraines in order to benefit from these medications. Some preliminary research indicates that venlafaxine (effexor) could also be beneficial. Current data does not support use of other antidepressants. Thanks to d. Jones.