Each illness has its own symptoms, but common signs of mental illness in adults and adolescents can include the following:

1. Excessive worrying or fear

2. Feeling excessively sad or low

3. Confused thinking or problems in concentrating or  learning

4. Extreme mood changes, including uncontrollable “highs” or feelings of euphoria

5. Prolonged or strong feelings of irritability or anger

6. Avoiding friends and social activities

7. Difficulties understanding or relating to other people

8. Changes in sleeping habits or feeling tired and low energy

9. Changes in eating habits such as increased hunger or lack of appetite

10. Difficulty perceiving reality (delusions or hallucinations, in which a person experiences and senses things that don't exist in objective reality)

11. Inability to perceive changes in one’s own feelings, behavior or personality (”lack of insight” or anosognosia)

12. Overuse of substances like alcohol or drugs