Because antidepressants work by altering some of the chemicals in the brain to try and offset some of the typical effects of depression. So there is a chance that if a person hits on the right antidepressant, they maybe able to offset 'some" of the problems mercury is causing, but it will not cure them of course.

Problem is, mercury also causes a person to react adversely very easily to things, so most do not tolerate many antidepressants, (if any at all).