Antidepressants can exert their effect on the brain only after passing the blood-brain barrier which protects the brain against potentially harmful substances which may be found in blood. The barrier is selective for transport of many medications from the blood into the brain because it contains transport molecules which remove these medications from the brain. The ABCB1 gene encodes one such transport molecule.
Even a small change in this gene results in a faster clearance of several antidepressants from the brain and can thus be responsible for ineffectiveness of these medications. Perhaps you would like to know whether you are also resistant to antidepressants?
More information about genetic testing for medications.