Major Depression is the common expression. But it is not always the one that the person has. When the person has this type of depression it usually lasts six months or more. It is often caused by the death of a loved one or other loss, but it can also have chemical causes within the brain. When a person feels down for months, it is best to consult a counselor and describe the symptoms. Anti-depressants are most often used to treat this longer term mood change. It is not best to ask you family doctor for the anti-depressant before consulting with a counselor. Many times the depression is caused by a mental illness like the Bipolar Disorder and a regular antidepressant would cause the person to become manic or suicidal.