The exact cause of depression is unknown. The brain makes chemicals, called neurotransmitters, which relay messages between brain cells. Those who have depression tend to have abnormal levels of certain neurotransmitters in the brain.

Neurotransmitter levels are affected by:

• Heredity
• Medications
• Personal tragedy
• Personality
• Substance abuse
• Illness

Types of depression include:

• Major depression
• Situational depression
• Teenage depression
• Organic affective disorder
• Bipolar disorder
• Dysthymic disorder
• Seasonal affective disorder
• Grief