How logically rigorous is the claim that neurochemical changes in the brain

How logically rigorous is the claim that neurochemical changes in the brain

How logically rigorous is the claim that neurochemical changes in the brain 'cause' mood or emotional disorders? Does a running nose cause a cold? In any case, before prescribing powerful chemicals to emotionally distressed patients shouldn't doctors use some sort of machine to test the chemical levels of their brains?

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