The brain adapts to psychiatric medications like antidepressants in order to maintain homeostasis (physiological balance). For example, when it is too hot, the body cools us down, and when it is too cold the body tries to warm us up.
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There is evidence of brain cell adaptation (particularly in serotonin receptors) in people starting after just a few weeks of antidepressant treatment.* ** Other studies in humans* and animals*** show that these effects can persist for months and years after antidepressants are stopped. The risk of withdrawal symptoms appears to increase with long-term use.

* Bhagwagar Z, Rabiner EA, Sargent PA, et al. Persistent reduction in brain serotonin1A receptor binding in recovered depressed men measured by positron emission tomography with [11C]WAY-100635. Mol Psychiatry. 2004;9(4):386-392.

** Haahr ME, Fisher PM, Jensen CG, et al. Central 5-HT4 receptor binding as biomarker of serotonergic tonus in humans: A [11C]SB207145 PET study. Mol Psychiatry.

*** Renoir T. Selective serotonin reuptake inhibitor antidepressant treatment discontinuation syndrome: A review of the clinical evidence and the possible mechanisms involved. Front Pharmacol. 2013;4 APR(April):1-10.014;19(4):427-432.