Obesity is the condition of having an excessive accumulation of fat in the body, resulting in a body weight more than 20% above the average for height, age, sex, and body type, and in elevated risk of disability, illness, and death.


An obsession is an unwelcome, uncontrollable, and persistent idea, thought, image, or emotion that a person cannot help thinking even though it creates significant distress or anxiety.

Obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is currently classified as an anxiety disorder marked by the recurrence of intrusive or disturbing thoughts, impulses, images or ideas (obsessions) accompanied by repeated attempts to suppress these thoughts through the performance of certain irrational and ritualistic behaviors or mental acts (compulsions). The obsessions and compulsions take up large amounts of the patient's time (an hour or longer every day) and usually cause significant emotional distress for the patient and difficulties in his or her relationships with others.

Obsessive-compulsive personality disorder

Obsessive-compulsive personality disorder (OCPD) is a type of personality disorder marked by rigidity, control, perfectionism, and an overconcern with work at the expense of close interpersonal relationships. Persons with this disorder often have trouble relaxing because they are preoccupied with details, rules, and productivity.


Olanzapine is classified as an atypical antipsychotic drug. It is available in the United States under the brand names Zyprexa and Zyprexa Zydis.

Opioids and related disorders

Opioids are a class of drugs that include both natural and synthetic substances. The natural opioids (referred to as opiates) include opium and morphine.

Oppositional defiant disorder

Oppositional defiant disorder (ODD) is a disorder found primarily in children and adolescents. It is characterized by negative, disobedient, or defiant behavior that is worse than the normal "testing" behavior most children display from time to time.

Origin of mental illnesses

Over the history of the healing arts, there has been an evolution of theories regarding the root causes of mental illness. Early writings from such ancient civilizations as those of Greece, Rome, India, and Egypt focused on demonic possession as the cause.


Oxazepam is a member of a family of tranquilizers known as benzodiazepines. It is sold in the United States under the brand name Serax and in Canada under the brand name Ox-Pam.

Pain disorder

Pain disorder is one of several somatoform disorders described in the revised, fourth edition of the mental health professional's handbook, the Diagnostic and Statistical Manual of Mental Disorders(known asDSM-IV-TR). The term "somatoform" means that symptoms are physical but are not entirely understood as a consequence of a general medical condition or as a direct effects of a substance, such as a drug.

Panic attack

Panic attacks, the hallmark of panic disorder, are discrete episodes of intense anxiety. Panic attacks can also be experienced by people with specific phobia, social phobia, or by people who have used or consumed certain substances, such as cocaine.

Panic disorder

Panic disorder is a condition in which the person with the disorder suffers recurrent panic attacks. Panic attacks are sudden attacks that are not caused by a substance (like caffeine), medication, or by a medical condition (like high blood pressure), and during the attack, the sufferer may experience sensations such as accelerated or irregular heartbeats, shortness of breath, dizziness, or a fear of losing control or "going crazy." The sudden attack builds quickly (usually within 10 minutes) and is almost paralyzing in its severity.


Paranoia is a symptom in which an individual feels as if the world is "out to get" him or her. When people are paranoid, they feel as if others are always talking about them behind their backs.

Paranoid personality disorder

People with paranoid personality disorder (PPD) have long-term, widespread and unwarranted suspicions that other people are hostile, threatening or demeaning. These beliefs are steadfastly maintained in the absence of any real supporting evidence.


Paraphilias are sexual feelings or behaviors that may involve sexual partners that are not human, not consenting, or that involve suffering by one or both partners.

Parent management training

Parent management training (PMT) is an adjunct to treatment that involves educating and coaching parents to change their child's problem behaviors using principles of learning theory and behavior modification.


Paroxetine is an antidepressant of the type known as selective serotonin reuptake inhibitors (SSRI). It is sold in the United States under the brand name Paxil.


Passionflower (Passiflora incarnata) is a vine whose leaves and flowers are widely used in Europe to make a herbal remedy for anxiety and insomnia. The plant, which is native to the tropical regions of North America, was first used by the Aztecs of Mexico as a folk remedy for these conditions.

Pathological gambling disorder

Pathological gambling disorder occurs when a person gambles compulsively to such an extent that the wagering has a severe negative effect on his or her job, relationships, mental health, or other important aspects of life. The person may continue to gamble even after they have developed social, economic, interpersonal, or legal problems as a result of the gambling.


Pedophilia is a paraphilia that involves an abnormal interest in children. A paraphilia is a disorder that is characterized by recurrent intense sexual urges and sexually arousing fantasies generally involving: nonhuman objects; the suffering or humiliation of oneself or one's partner (not merely simulated); or animals, children, or other nonconsenting persons.

Peer groups

Peer groups are an important influence throughout one's life, but they are more critical during the developmental years of childhood and adolescence. There is often controversy about the influence of a peer group versus parental influence, particularly during adolescence.


Pemoline is classified as a central nervous system (CNS) stimulant. It is sold in the United States under the brand names Cylert and PemADD and is also available under its generic name.


Perphenazine is a phenothiazine antipsychotic used to treat serious mental disorders. It has also been used to treat severe nausea and vomiting.

Personality disorders

Long-standing, deeply ingrained patterns of social behavior that are detrimental to those who display them or to others.

Person-centered therapy

Person-centered therapy, which is also known as client-centered, non-directive, or Rogerian therapy, is an approach to counseling and psychotherapy that places much of the responsibility for the treatment process on the client, with the therapist taking a nondirective role.

Pervasive developmental disorders

Pervasive developmental disorders are a group of conditions originating in childhood that involve serious impairment in several areas, including physical, behavioral, cognitive, social, and language development.

Phencyclidine and related disorders

Phencyclidine (PCP) is a street drug known as "angel dust" that causes physiological changes to the nervous and circulatory system, disturbances in thinking and behavior, and can cause hallucinations, psychotic disorder, mood disorder, and anxiety disorder.


Phenelzine is classified as a monoamine oxidase (MAO) inhibitor. In the United States, phenelzine is sold under the brand name Nardil.

Phonological disorder

Phonological disorder occurs when a child does not develop the ability to produce some or all sounds necessary for speech that are normally used at his or her age.


Pica is a term that refers to cravings for substances that are not foods. Materials consumed by patients with pica include dirt, ice, clay, glue, sand, chalk, beeswax, chewing gum, laundry starch, and hair.


Pimozide is an atypical antipsychotic drug used to treat serious motor and verbal tics associated with Tourette's syndrome. It is sold under the brand name Orap.

Play therapy

Play therapy refers to a method of psychotherapy with children in which a therapist uses a child's fantasies and the symbolic meanings of his or her play as a medium for understanding and communication with the child.


Polysomnography is a series of tests performed on patients while they sleep. Polysomnography is a comprehensive overnight procedure that evaluates sleep disorders.

Polysubstance dependence

Polysubstance dependence refers to a type of substance dependence disorder in which an individual uses at least three different classes of substances indiscriminately and does not have a favorite drug that qualifies for dependence on its own.

Positive symptoms

Positive symptoms are thoughts, behaviors, or sensory perceptions present in a person with a mental disorder, but not present in people in the normal population.

Positron emission tomography

Positron emission tomography (PET) is a highly specialized imaging technique using short-lived radiolabeled substances to produce extremely high resolution images of the body's biological function.

Postpartum depression

Postpartum depression is a depression that can range from mild to suicidal and can occur anytime after delivery up to one year later.

Post-traumatic stress disorder

Post-traumatic stress disorder, often abbreviated as PTSD, is a complex disorder in which the affected person's memory, emotional responses, intellectual processes, and nervous system have all been disrupted by one or more traumatic experiences. It is sometimes summarized as "a normal reaction to abnormal events." The DSM-IV-TR(the professional's diagnostic manual) classifies PSTD as an anxiety disorder.

Premature ejaculation

Premature ejaculation (PE) refers to the persistent or recurrent discharge of semen with minimal sexual stimulation before, on, or shortly after penetration, before the person wishes it, and earlier than he expects it. In making the diagnosis of PE, the clinician must take into account factors that affect the length of time that the man feels sexually excited.


Propranolol is classified as a beta blocker. It is sold in the United States under the brand name Inderal.


Protriptyline is an oral tricyclic antidepressant. It is sold in the United States under the brand name Vivactil and is also available under its generic name.


Pseudocyesis is the medical term for a false pregnancy. Pseudocyesis can cause many of the signs and symptoms of pregnancy, and often resembles the condition in every way except for the presence of a fetus.


A psychiatrist is a physician who specializes in the diagnosis and treatment of mental disorders.


Psychoanalysis, as a form of therapy, is based on the understanding that human beings are largely unaware of the mental processes that determine their thoughts, feelings, and behavior, and that psychological suffering can be alleviated by making those processes known to the individual.

Psychodynamic psychotherapy

Psychodynamic psychotherapy is a method of verbal communication used to help a person find relief from emotional pain. It is based on the theories and techniques of psychoanalysis.


A psychologist is a social scientist who studies behavior and mental processes, generally in a research or clinical setting.


Psychosis is a symptom of mental illness characterized by a radical change in personality and a distorted or diminished sense of objective reality.


Psychosurgery is the treatment of a psychiatric disorder using surgical techniques to destroy brain tissue and is now rarely used.


The treatment of mental or emotional disorders and adjustment problems through the use of psychological techniques rather than through physical or biological means.

Psychotherapy integration

Psychotherapy integration is defined as an approach to psychotherapythat includes a variety of attempts to look beyond the confines of single-school approaches in order to see what can be learned from other perspectives. It is characterized by an openness to various ways of integrating diverse theories and techniques.