SUNSET461 writes: "If anyone has been on a certain antidepressant for a period of time, which was helping them a lot with their depression, but suddently they found that it stopped working, they suffered from a phenomenon called: 'Developing drug tolerance'. This is how it happens. After taking a certain amount of antidepressant medication over a period of time, the receptors located at the ends of the nerve cells (neurons) in the brain become saturated with certain hormones/neurotransmitters, from the continuous use of the medication. This causes the number of receptors to decrease in order to compensate for the surplus. Also, the remaining receptors absorb less neurotransmitters.
Once there are less receptors to pass on the neurotransmitters (like the norepinephrine and dopamine) to the next neuron's receptors -- as this is how cells communicate with each other and keep the brain functioning normally -- The result is that not enough medication is being absorbed. This is called developing a tolerance to a medication. So by stopping a medication for a while until the brain readjusts, then starting again, can make the medication work once more. Also switching medications for a while, or adding another antidepressant to the first one (called augmentation). I found an article on one of the three brain neurotransmitters, dopamine, that explains how tolerance occurs: http://www.utexas.edu/research/asrec/dopamine.html"