The most common class of antidepressant, SSRIs, don’t work the way scientists previously thought. A brain-scan study of 22 healthy individuals who had no history of depression, anxiety, or SSRI use showed that the drugs rewire the brain almost instantly. A single dose produced a lasting change in brain structure that was easily measured when researchers checked after three hours. This seems to contradict clinical evidence that suggests the drug effects become evident only after dozens of doses taken over a period of weeks. More perplexing, the brain scans mainly showed the SSRIs caused a reduction in brain energy and activity in almost every area of the brain, which is not the way antidepressants were thought to work.
This is only a very preliminary look at the way SSRIs work, but one scenario that has to be considered is that SSRIs have most of their beneficial effect in the first dose, with subsequent doses adding only the slightest additional drug effect. This might be seen as a worst-case scenario for drug manufacturers. Their revenue from SSRIs could fall by 98 percent if doctors conclude they can prescribe just one tablet for each patient.
The research is summarized at Time: