Clinical use of most of the anti-depressants of all classes is usually based on empiric effect. That is, one starts the drug and watches the effect on depression. The drug is increased until side-effects appear, or the depression is treated. There are a few of the anti-depressants which do have blood levels available, but there are only a few. Consequently, if the depression appears to be increasing it certainly would make sense to increase the dose and watch the effect(vs. side-effects). There doesn't appear to be major tolerance with long-term use.
However, on the other hand people have experienced a decreased efficacy with long term use, and have tried many things in consultation with their doctors, please read the link below in details for the alternative various people have tried:
http://www.dr-bob.org/babble/20021122/msgs/129427.html
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regards
Dr. Gupta

