Psychological Mechanism of Action
Cognitive behavior therapy (CBT) is a structured form of psychotherapy in which the client is taught to recognize and change thought processes that cause or contribute to the deepening of symptoms of depression. For example, in this case, when Peggy felt worthless as a mother and a student her symptoms of depression intensified (she ran off to the restroom in tears). In cognitive therapy she would be taught to recognize this as "all or nothing thinking" or an "overgeneralization." She would be encouraged in therapy to challenge this distortion with more realistic thoughts like, "Just because I'm having problems coping at this time, doesn't mean I'm 'totally worthless.' In fact I am able to help my son get food and I am attending classes and doing some homework." Most patients experience some lifting of the intensity of their depression when they are able to do this type of challenging. This often leads to being able to cope more effectively, which can lift the depression even further. Clients are also taught skills for dealing with things like poor concentration and procrastination that also help relieve dysphoric feelings.

CBT was developed and studied by Aaron Beck and his group of therapist-researchers at the University of Pennsylvania. It was compared with interpersonal psychotherapy (IPT) and imipramine (the main tricyclic antidepressant drug in use at the time) in a major study conducted by the National Institute of Mental Health in 1977.

Indications
CBT was found to be as effective as imipramine and IPT in the treatment of depression. All three treatments were equivalent in the degree of success and for the most part superior to the placebo control group. Imipramine was somewhat faster in reducing symptoms, but by the end of the 16 weeks of treatments, the three groups were equivalent.

On average it may take 3-7 weeks or more for the person to gain enough symptom relief to make a positive difference with CBT.

Some analyses of the data showed that the drug was more effective with severely depressed individuals. Several later studies have disputed this finding. Some studies have shown that patients receiving CBT have been less likely to relapse though there is not clear agreement on this finding. A few studies have shown that cognitive therapy helps make standard antidepressants more effective (probably by reducing the drop-out rate in the antidepressant groups) but most of these studies have failed to show an advantage for combined treatment.

Side Effects
Though there are no obvious side effects with CBT, there are some risks and difficulties. First one needs to be in therapy from 16 to 20 weeks. Finding an hour or more a week and locating a therapist in reasonable driving distance also can be complicating factors. Some research has shown that compatibility with the therapist is a factor in treatment success. It may take some trial and error to find the right therapist and if the therapy is disrupted the client may experience this as a significant "loss" and thus have to deal with a deepened depression.

Cost
Sometimes, especially in the beginning, the person needs to go for two therapy sessions a week. Since most therapists now charge between $90 and $120 per session, the cost can be a concern. Over a one-month period, the cost can be between $360 and $480. Some health insurance programs cover costs for therapy but few of them cover the full cost and almost none allow an unlimited number of visits. College and university counseling centers often offers diagnosis and this type of therapy free for a limited number of sessions if it is deemed appropriate.

Resource:  http://www.apa.org/journals/anton.html