Mood disorders, like high blood pressure and diabetes, require maintenance medication. After a course of treatment, the doctor will evaluate the patient's progress and the dosage of medication can be reduced or even no longer required.
Of course you don't need to take medicine if you are not sick. Mild illnesses can receive cognitive behavioral therapy, and moderate or above illnesses generally require drug treatment.
If psychological treatment is sufficient, drugs are certainly not needed; but if drug treatment is additionally helpful to the patient, drugs combined with psychotherapy can be more effective.
