This article is from the Depression FAQ, by Cynthia Frazier with numerous contributions by others.
Tolerance to Prozac and the other SSRIs is a relatively rare phenomenon. What looks like tolerance may develop because the SSRIs also have effects on the dopamine systems of the brain, and these effects can slow one down dramatically.
When an SSRI sems not to be working as well as it once did, it often can be helped to work once again by adding small doses of a dopaminergic agonist such as dextrroamphetamine, Ritalin, or bromocriptene. Also, certainly with Proxzac, and possibly with other SSRIs, too much of the drug is as ineffective as too little. If raising the dose does not help, an certainly if it makes things worse, a lowering of the dose may do much to bring back a response.
I am convinced that many patients respond best is they are treated with one of the SSRIs + a tricyclic antidepressant such as desipramine (Norpramin), or nortriptyline (Aventyl). Such combinations are often effective when an SSRI by itself fails to do the job