With regard to the efficacy over placebos, I can't speak to it more broadly, but the low dose antidepressant I'm now was after some trial and error. Observation of its effect on me, potential side-effects and adjusting the dosage. While its impossible to genuinely know if any one individual case is a consequence of the placebo effect or not but its not being prescribed to me pro-forma. 

There's a delicately balanced cocktail I'm on, calibrated to prevent grand falls into suicidal depression, but still allow for mood to fluctuate enough for zealous creativity and passion, accepting the weariness that comes after it.