Trichophagia is characterized by the person eating hair, usually their own; primarily after pulling it out. Most often, hair is pulled out and then the ends of the root bulb are eaten, or occasionally the hair shaft itself. The hair eventually collects in the gastrointestinal tract (on occasion, and depending upon severity of symptoms) causing indigestion and stomach pain. Ritual is a strong factor, and may involve touching the root bulb to the lips, tasting the hair, and occasionally chewing it. Sometimes those with the disorder may even eat the hair of others. In the psychiatric field it is considered a compulsive psychological disorder.

Trichophagia () is the compulsive eating of hair associated with trichotillomania (hair pulling). In trichophagia, people with trichotillomania also ingest the hair that they pull; in extreme (and rare) cases this can lead to a hair ball (trichobezoar).

Trichotillomania is defined as a self-induced and recurrent loss of hair. It includes the criterion of an increasing sense of tension before pulling the hair and gratification or relief when pulling the hair. However, some people with trichotillomania do not endorse the inclusion of "rising tension and subsequent pleasure, gratification, or relief" as part of the criteria; because many individuals with trichotillomania may not realize they are pulling their hair, patients presenting for diagnosis may deny the criteria for tension prior to hair pulling or a sense of gratification after hair is pulled.

Trichotillomania may lie on the obsessive-compulsive spectrum, also encompassing obsessive-compulsive disorder (OCD), body dysmorphic disorder (BDD), nail biting (onychophagia) and skin picking (dermatillomania), tic disorders and eating disorders. These conditions may share clinical features, genetic contributions, and possibly treatment response; however, differences between trichotillomania and OCD are present in symptoms, neural function and cognitive profile. In the sense that it is associated with irresistible urges to perform unwanted repetitive behavior, trichotillomania is akin to some of these conditions, and rates of trichotillomania among relatives of OCD patients is higher than expected by chance. However, differences between the disorder and OCD have been noted, including: differing peak ages at onset, rates of comorbidity, gender differences, and neural dysfunction and cognitive profile. When it occurs in early childhood, it can be regarded as a distinct clinical entity.

Because trichotillomania can be present in multiple age groups, it is helpful in terms of prognosis and treatment to approach three distinct subgroups by age: preschool age children, preadolescents to young adults, and adults.

Trichotillomania is often not a focused act, but rather hair pulling occurs in a "trance-like" state; hence, trichotillomania is subdivided into "automatic" versus "focused" hair pulling. Children are more often in the automatic, or unconscious, subtype and may not consciously remember pulling their hair. Other individuals may have focused, or conscious, rituals associated with hair pulling, including seeking specific types of hairs to pull, pulling until the hair feels "just right", or pulling in response to a specific sensation. Knowledge of the subtype is helpful in determining treatment strategies.

Trichotillomania is usually confined to one or two sites, but can involve multiple sites. The scalp is the most common pulling site, followed by the eyebrows, eyelashes, face, arms, and legs. Some less common areas include the pubic area, underarms, beard, and chest. The classic presentation is the "Friar Tuck" form of vertex and crown alopecia. Children are less likely to pull from areas other than the scalp.

People who suffer from trichotillomania often pull only one hair at a time and these hair-pulling episodes can last for hours at a time. Trichotillomania can go into remission-like states where the individual may not experience the urge to "pull" for days, weeks, months, and even years.

Individuals with trichotillomania exhibit hair of differing lengths; some are broken hairs with blunt ends, some new growth with tapered ends, some broken mid-shaft, or some uneven stubble. Scaling on the scalp is not present, overall hair density is normal, and a hair pull test is negative (the hair does not pull out easily). Hair is often pulled out leaving an unusual shape. Individuals with trichotillomania may be secretive or shameful of the hair pulling behavior.

An additional psychological effect can be low self-esteem, often associated with being shunned by peers and the fear of socializing, due to appearance and negative attention they may receive. Some people with trichotillomania wear hats, wigs, false eyelashes, eyebrow pencil, or style their hair in an effort to avoid such attention. There seems to be a strong stress-related component. In low-stress environments, some exhibit no symptoms (known as "pulling") whatsoever. This "pulling" often resumes upon leaving this environment. Some individuals with trichotillomania may feel they are the only person with this problem due to low rates of reporting.

For some people, trichotillomania is a mild problem, merely a frustration. But for many, shame and embarrassment about hair pulling causes painful isolation and results in a great deal of emotional distress, placing them at risk for a co-occurring psychiatric disorder, such as a mood or anxiety disorder. Hair pulling can lead to great tension and strained relationships with family members and friends. Family members may need professional help in coping with this problem.

Other medical complications include infection, permanent loss of hair, repetitive stress injury, carpal tunnel syndrome, and gastrointestinal obstruction as a result of trichophagia. In trichophagia, people with trichotillomania also ingest the hair that they pull; in extreme (and rare) cases this can lead to a hair ball (trichobezoar). Rapunzel syndrome, an extreme form of trichobezoar in which the "tail" of the hair ball extends into the intestines, can be fatal if misdiagnosed.

Environment is a large factor which affects hair pulling. Sedentary activities such as being in a relaxed environment are conducive to hair pulling. A common example of a sedentary activity promoting hair pulling is lying in a bed while trying to rest or fall asleep. An extreme example of automatic trichotillomania is found when some patients have been observed to pull their hair out while asleep. This is called sleep-isolated trichotillomania.

Dermatophagia sufferers chew their skin out of compulsion, and can do so on a variety of places on their body. Sufferers typically chew the skin surrounding their fingernails and joints. They also chew on the inside of their mouth, cheeks, and/or lips, causing blisters in and outside of the mouth. If the behavior is left unchecked for an extended period, calluses may start to develop where most of the biting is done.

Skin chewing can be bolstered by times of apprehension and other unpleasant events. Blisters in particular can cause a feeling of desire to pull or bite off the affected skin (since the skin is dead, thus easily pulled off), which could be detrimental, causing infection. Another disorder, known as dermatillomania , the act of picking at one's skin, can sometimes accompany dermatophagia. People who suffer from dermatophagia can also be prone to infection as when they bite their fingers so frequently, they make themselves vulnerable to bacteria seeping in and causing infection. Dermatophagia can be considered a "sister" disorder to trichophagia, which involves compulsively biting and eating one's hair.

Dermatophagia (from Ancient Greek "δέρμα" - skin - and "φαγεία" - eating) is a compulsion of gnawing or eating one's own skin, most commonly at the fingers. Sufferers typically bite the skin around the nails, leading to bleeding and discoloration over time. Some may consume the flesh during an episode.

Contemporary research suggests a link between impulse control disorders and obsessive–compulsive disorders, and this may be addressed in the "DSM-5", published in May 2013. Further information on OCD, other anxiety disorders, and dermatophagia and other impulse-control disorders can be found in the "DSM-IV TR".

Pica is characterized by an appetite for substances that are largely non-nutritive, such as ice (pagophagia); hair (trichophagia); paper (xylophagia); drywall or paint; metal (metallophagia); stones (lithophagia) or soil (geophagia); glass (hyalophagia); or feces (coprophagia); and chalk. According to DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, 4th Edition) criteria, for these actions to be considered pica, they must persist for more than one month at an age where eating such objects is considered developmentally inappropriate, not part of culturally sanctioned practice and sufficiently severe to warrant clinical attention. It can lead to intoxication in children, which can result in an impairment of both physical and mental development. In addition, it can also lead to surgical emergencies due to an intestinal obstruction as well as more subtle symptoms such as nutritional deficiencies and parasitosis. Pica has been linked to other mental and emotional disorders. Stressors such as emotional trauma, maternal deprivation, family issues, parental neglect, pregnancy, and a disorganized family structure are strongly linked to pica as a form of comfort.

Pica is most commonly seen in pregnant women, small children, and those with developmental disabilities such as autism. Children eating painted plaster containing lead may suffer brain damage from lead poisoning. There is a similar risk from eating soil near roads that existed before tetraethyllead in petrol was phased out (in some countries) or before people stopped using contaminated oil (containing toxic PCBs or dioxin) to settle dust. In addition to poisoning, there is also a much greater risk of gastro-intestinal obstruction or tearing in the stomach. Another risk of eating soil is the ingestion of animal feces and accompanying parasites. Pica can also be found in other animals and is commonly found in dogs.

The main BFRB disorders are:

- Skin

- Dermatillomania (excoriation disorder), skin picking

- Dermatophagia, skin nibbling

- Mouth

- Morsicatio buccarum, cheek biting

- Morsicatio labiorum, inner lip biting

- Morsicatio buccarum, tongue biting

- Hands

- Onychophagia, nail biting

- Onychotillomania, nail picking

- Nose

- Rhinotillexomania, compulsive nose picking

- Hair

- Trichophagia, hair nibbling

- Trichotemnomania, hair cutting

- Trichotillomania, hair pulling

Complications may occur due to the substance consumed. For example, lead poisoning may result from the ingestion of paint or paint-soaked plaster, hairballs may cause intestinal obstruction and "Toxoplasma" or "Toxocara" infections may follow ingestion of feces or dirt.

Body-focused repetitive behavior (BFRB) is an umbrella name for impulse control behaviors involving compulsively damaging one's physical appearance or causing physical injury.

Body-focused repetitive behavior disorders (BFRBDs) in ICD-11 is in development.

BFRB disorders are not generally considered obsessive-compulsive disorders.