Depression is a mood disorder that causes a persistent feeling of sadness and loss of interest. Also called major depressive disorder or clinical depression, it affects how you feel, think and behave and can lead to a variety of emotional and physical problems. You may have trouble doing normal day-to-day activities, and sometimes you may feel as if life isn't worth living.

It's normal to feel anxious from time to time, especially if your life is stressful. However, excessive, ongoing anxiety and worry that are difficult to control and interfere with day-to-day activities may be a sign of generalized anxiety disorder. Generalized anxiety disorder has symptoms that are similar to panic disorder, obsessive-compulsive disorder and other types of anxiety, but they're all different conditions.

(ADHD) is a mental health disorder that includes a combination of persistent problems, such as difficulty paying attention, hyperactivity and impulsive behavior.  In some cases, ADHD is not recognized or diagnosed until the person is an adult. 

Attention-deficit/hyperactivity disorder (ADHD) is a chronic condition that affects millions of children and often continues into adulthood. ADHD includes a combination of persistent problems, such as difficulty sustaining attention, hyperactivity and impulsive behavior.

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Schizophrenia is a serious mental disorder in which people interpret reality abnormally. Schizophrenia may result in some combination of hallucinations, delusions, and extremely disordered thinking and behavior that impairs daily functioning and can be disabling.

People with schizophrenia require lifelong treatment. 

Childhood schizophrenia is an uncommon but severe mental disorder in which children interpret reality abnormally. Schizophrenia involves a range of problems with thinking (cognitive), behavior or emotions. It may result in some combination of hallucinations, delusions, and extremely disordered thinking and behavior that impairs your child's ability to function.

Schizoaffective disorder is a mental disorder in which a person experiences a combination of schizophrenia symptoms, such as hallucinations or delusions, and mood disorder symptoms, such as depression or mania. The two types of schizoaffective disorder are:

• Bipolar type

• Depressive type

Bipolar disorder, formerly called manic depression, is a mental health condition that causes extreme mood swings that include emotional highs (mania or hypomania) and lows (depression).

These mood swings can affect sleep, energy, activity, judgment, behavior and the ability to think clearly.

A personality disorder is a type of mental disorder in which you have a rigid and unhealthy pattern of thinking, functioning and behaving. A person with a personality disorder has trouble perceiving and relating to situations and people. This causes significant problems and limitations in relationships, social activities, work and school.

Post-traumatic stress disorder (PTSD) is a mental health condition that's triggered by a terrifying event — either experiencing it or witnessing it. Symptoms may include flashbacks, nightmares and severe anxiety, as well as uncontrollable thoughts about the event.

Obsessive-compulsive disorder (OCD) features a pattern of unreasonable thoughts and fears (obsessions) that lead you to do repetitive behaviors (compulsions). These obsessions and compulsions interfere with daily activities and cause significant distress.

Dementia describes a group of symptoms affecting memory, thinking and social abilities severely enough to interfere with your daily life. It isn't a specific disease, but several different diseases may cause dementia.

Though dementia generally involves memory loss, memory loss has different causes. Having memory loss alone doesn't mean you have dementia.

Drug addiction, also called substance use disorder, is a disease that affects a person's brain and behavior and leads to an inability to control the use of a legal or illegal drug or medication. Substances such as alcohol, marijuana and nicotine also are considered drugs. When you're addicted, you may continue using the drug despite the harm it causes.

Alcohol use disorder (which includes a level that's sometimes called alcoholism) is a pattern of alcohol use that involves problems controlling your drinking, being preoccupied with alcohol, continuing to use alcohol even when it causes problems, having to drink more to get the same effect, or having withdrawal symptoms.

Nicotine dependence ― also called tobacco dependence ― is an addiction to tobacco products caused by the drug nicotine. Nicotine dependence means you can't stop using the substance, even though it's causing you harm.

Nicotine produces physical and mood-altering effects in your brain that are temporarily pleasing.