Part of the reason for the disparity is that people's genetics can drastically influence what effect particular drugs have. Switching drugs is a pain as one is

Part of the reason for the disparity is that people's genetics can drastically influence what effect particular drugs have. Switching drugs is a pain as one is supposed to taper off the old one, then switch to the new one and wait 4-12 weeks for the new one to take effect. If one drug works "good enough" then you're unlikely to want to even try a different one. I finally took a DNA test last year after reading about it (a "pharmacodynamics" one that your doctor has to order). Reading the results showed that all of the antidepressants that I had tried in the past were ones that I either had a gene to metabolize too fast or another gene that said they'd be ineffective for me.