Hi, Mirtazapine belongs to the group of antidepressant medications known as tetracyclic antidepressants that is used in the treatment of depression. But like other antidepressants, it may stop working over time. If someone has been taking a particular antidepressant like Mirtazapine for a long time, then his or her body may develop tolerance and that particular antidepressant may become ineffective and simply stop working. He or she can notice some signs if his or her antidepressant has stopped working such as no relief from the symptoms of depression, worsening of depression, violent mood swings, overwhelming of the antidepressant s side effects like painful reddening of the skin or blisters or sores or impetigo on the body or in the mouth etc. If someone experiences such type of symptoms, then he or she should inform his or her prescribing doctor promptly and the doctor may increase the dose or switch to or add another antidepressant. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Zubayer Alam, General And Family Physician