Currently the way doctors prescribe psychiatric meds is pretty much the eeny, meeny, miny, moe way: We start with a little bit of this, we add a little bit of that and so on. This is because, with a few clinical exceptions, there is little evidence to favor one treatment over another for a given patient. This is changing however.
A recent study published in JAMA using PET scans, showed marked differences in brain activity in patients that responded better to cognitive behavior therapy as opposed to Lexapro. Patients with low activity in the brain region called anterior insula responded quite well to behavior therapy but poorly to Lexapro.
In the near future scans will be able to tell us which therapy will be more effective with what mental conditions.
Another large, multicenter study conducted by Dr. Nemeroff, professor of psychiatry at Emory, found that in patients with a history of childhood trauma 48% achieved remission with cognitive therapy alone while only 33% responded to drugs alone. The combination of drugs and therapy was not significantly better than therapy alone. Considering the high rate of trauma and early childhood abuse in depressed people doctors will serve their patients better if they educate them about the benefits of therapy and encourage them to pursue it.