Muscle dysmorphia is a subtype of the obsessive mental disorder body dysmorphic disorder, but is often also grouped with eating disorders. In muscle dysmorphia, which is sometimes called "bigorexia", "megarexia", or "reverse anorexia", the delusional or exaggerated belief is that one's own body is too small, too skinny, insufficiently muscular, or insufficiently lean, although in most cases, the individual's build is normal or even exceptionally large and muscular already.

Muscle dysmorphia affects mostly men, particularly those involved in sports where body size or weight are competitive factors, becoming rationales to gain muscle or become leaner. The quest to seemingly fix one's body consumes inordinate time, attention, and resources, as on exercise routines, dietary regimens, and nutritional supplementation, while use of anabolic steroids is also common. Other body-dysmorphic preoccupations that are not muscle-dysmorphic are usually present as well.

Although likened to anorexia nervosa, muscle dysmorphia is especially difficult to recognize, since awareness of it is scarce and persons experiencing muscle dysmorphia typically remain healthy looking. The distress and distraction of muscle dysmorphia may provoke absences from school, work, and social settings. Compared to other body dysmorphic disorders, rates of suicide attempts are especially high with muscle dysmorphia. Researchers believe that muscle dysmorphia's incidence is rising, partly due to the recent cultural emphasis on muscular male bodies.

Although body dissatisfaction has been found in boys as young as age six, muscle dysmorphia's onset is estimated at usually between ages 18 and 20. According to DSM-5, muscle dysmorphia is indicated by the diagnostic criteria for body dysmorphic disorder via "the idea that his or her body is too small or insufficiently muscular", and this specifier holds even if the individual is preoccupied with other body areas, too, as is often the case.

Further clinical features identified include excessive conduct of efforts to increase muscularity, activities such as dietary restriction, overtraining, and injection of growth-enhancing drugs. Persons experiencing muscle dysmorphia generally spend over three hours daily pondering increased muscularity, and may feel unable to limit weightlifting. As in anorexia nervosa, the reverse quest in muscle dysmorphia can be insatiable. Those suffering from the disorder closely monitor their body and may wear multiple clothing layers to make it appear larger.

Muscle dysmorphia involves severe distress at having one's body viewed by others. Occupational and social functioning are impaired, and dietary regimes may interfere with these. Patients often avoid activities, people, and places that threaten to reveal their perceived deficiency of size or muscularity. Roughly half of patients have poor or no insight that these perceptions are unrealistic. Patient histories reveal elevated rates of diagnoses of other mental disorders, including eating disorders, mood disorders, anxiety disorders, and substance use disorder, as well as elevated rates of suicide attempts.

Although muscle dysmorphia's development is unclear, several risk factors have been identified.

Versus the general population, persons manifesting muscle dysmorphia are more likely to have experienced or observed traumatic events like sexual assault or domestic violence, or to have sustained adolescent bullying and ridicule for actual or perceived deficiencies such as smallness, weakness, poor athleticism, or intellectual inferiority. Increased body mass may seem to reduce the threat of further mistreatment.