that's what I understand about an antidepressant working, that everyone's biochem is just a little different, including how the drug is processed in the body, etc, etc
So, zoloft might work great for one person, not do much for another. But this second person might be helped by another antidepressant, even one in the same "class." And can take 4 to 8 weeks to tell, maybe more. So, in conclusion, the important thing is to stay a little loosey-goosey and be willing to move laterally.
Also it's often important to phase down in steps, even if the medicine doesn't seem to be working, just that your body may have gotten used to it.