Signs and symptoms of mental illness can vary, depending on the disorder, circumstances and other factors. Mental illness symptoms can affect emotions, thoughts and behaviors.

Examples of signs and symptoms include:

• Feeling sad or down
• Confused thinking or reduced ability to concentrate
• Excessive fears or worries, or extreme feelings of guilt
• Extreme mood changes of highs and lows
• Withdrawal from friends and activities
• Significant tiredness, low energy or problems sleeping
• Detachment from reality (delusions), paranoia or hallucinations
• Inability to cope with daily problems or stress
• Trouble understanding and relating to situations and to people
• Problems with alcohol or drug use
• Major changes in eating habits
• Sex drive changes
• Excessive anger, hostility or violence
• Suicidal thinking

Sometimes symptoms of a mental health disorder appear as physical problems, such as stomach pain, back pain, headaches, or other unexplained aches and pains.

Mental illnesses, in general, are thought to be caused by a variety of genetic and environmental factors:

• Inherited traits. Mental illness is more common in people whose blood relatives also have a mental illness. Certain genes may increase your risk of developing a mental illness, and your life situation may trigger it.
• Environmental exposures before birth. Exposure to environmental stressors, inflammatory conditions, toxins, alcohol or drugs while in the womb can sometimes be linked to mental illness.
• Brain chemistry. Neurotransmitters are naturally occurring brain chemicals that carry signals to other parts of your brain and body. When the neural networks involving these chemicals are impaired, the function of nerve receptors and nerve systems change, leading to depression and other emotional disorders.

Certain factors may increase your risk of developing a mental illness, including:

• A history of mental illness in a blood relative, such as a parent or sibling
• Stressful life situations, such as financial problems, a loved one's death or a divorce
• An ongoing (chronic) medical condition, such as diabetes
• Brain damage as a result of a serious injury (traumatic brain injury), such as a violent blow to the head
• Traumatic experiences, such as military combat or assault
• Use of alcohol or recreational drugs
• A childhood history of abuse or neglect
• Few friends or few healthy relationships
• A previous mental illness

Mental illness is common. About 1 in 5 adults has a mental illness in any given year. Mental illness can begin at any age, from childhood through later adult years, but most cases begin earlier in life.

The effects of mental illness can be temporary or long lasting. You also can have more than one mental health disorder at the same time. For example, you may have depression and a substance use disorder.

Mental illness is a leading cause of disability. Untreated mental illness can cause severe emotional, behavioral and physical health problems. Complications sometimes linked to mental illness include:

• Unhappiness and decreased enjoyment of life
• Family conflicts
• Relationship difficulties
• Social isolation
• Problems with tobacco, alcohol and other drugs
• Missed work or school, or other problems related to work or school
• Legal and financial problems
• Poverty and homelessness
• Self-harm and harm to others, including suicide or homicide
• Weakened immune system, so your body has a hard time resisting infections
• Heart disease and other medical conditions