About half of patients who are diagnosed with a Major Depressive Disorder have had significant symptoms prior to the first diagnosed episode.

In some the symptoms may be experienced fairly suddenly or acutely while in others there may be a long prodrome and it is only retrospectively that changes in mood, behaviour and functioning are recognised.

An untreated depressive episode lasts from 6 - 13 months with the average duration being around 9 months. Most cases will improve although a significant minority go on to develop a chronic depressive illness. Most treated episodes last about 3 months. However, medication should be continued for longer (6 - 9 months for a first episode) because withdrawal from medication too early is almost always associated with a relapse in the depressive symptoms.

As mentioned previously it is believed that the first episode in a mood disorder brings about long lasting changes, which increase susceptibility to subsequent episodes. It is also thought that if the initial episode is treated early enough, with adequate medication, for long enough some of these changes may be prevented.

About 5 - 10% of patients who have initially been diagnosed with a MDD will experience a manic episode 6 - 10 years after the first depressive episode. The average age for that switch is 32 years and it usually occurs after 2 - 4 episodes of depression.