Anorexia nervosa (AN), often referred to simply as anorexia, is an eating disorder characterized by food restriction, body image disturbance, fear of gaining weight, and an overpowering desire to be thin.

Individuals with anorexia nervosa have a fear of being overweight or being seen as such, despite the fact that they are typically underweight. The DSM-5 describes this perceptual symptom as "disturbance in the way in which one's body weight or shape is experienced". In research and clinical settings, this symptom is called "body image disturbance" or body dysmorphia. Individuals with anorexia nervosa also often deny that they have a problem with low weight due to their altered perception of appearance. They may weigh themselves frequently, eat small amounts, and only eat certain foods. Some patients with anorexia nervosa binge eat and purge to influence their weight or shape. Purging can manifest as induced vomiting, excessive exercise, and/or laxative abuse. Medical complications may include osteoporosis, infertility, and heart damage, along with the cessation of menstrual periods. Complications in men may include lowered testosterone. In cases where the patients with anorexia nervosa continually refuse significant dietary intake and weight restoration interventions, a psychiatrist can declare the patient to lack capacity to make decisions. Then, these patients' medical proxies decide that the patient needs to be fed by restraint via nasogastric tube.

Anorexia often develops during adolescence or young adulthood. One psychologist found multiple origins of anorexia nervosa in a typical female patient, but primarily sexual abuse and problematic familial relations, especially those of overprotecting parents showing excessive possessiveness over their children. The exacerbation of the mental illness is thought to follow a major life-change or stress-inducing events. Ultimately however, causes of anorexia are varied and differ from individual to individual. There is emerging evidence that there is a genetic component, with identical twins more often affected than fraternal twins. Cultural factors play a very significant role, with societies that value thinness having higher rates of the disease. Anorexia also commonly occurs in athletes who play sports where a low bodyweight is thought to be advantageous for aesthetics or performance, such as dance, gymnastics, running, figure skating and ski jumping (Anorexia athletica).

Treatment of anorexia involves restoring the patient back to a healthy weight, treating their underlying psychological problems, and addressing underlying maladaptive behaviors. A daily low dose of olanzapine has been shown to increase appetite and assist with weight gain in anorexia nervosa patients. Psychiatrists may prescribe their anorexia nervosa patients medications to better manage their anxiety or depression. Different therapy methods may be useful, such as cognitive behavioral therapy or an approach where parents assume responsibility for feeding their child, known as Maudsley family therapy. Sometimes people require admission to a hospital to restore weight. Evidence for benefit from nasogastric tube feeding is unclear. Some people with anorexia will have a single episode and recover while others may have recurring episodes over years. The largest risk of relapse occurs within the first year post-discharge from eating disorder therapy treatment. Within the first two years post-discharge, approximately 31% of anorexia nervosa patients relapse. Many complications, both physical and psychological, improve or resolve with nutritional rehabilitation and adequate weight gain.

It is estimated to occur in 0.3% to 4.3% of women and 0.2% to 1% of men in Western countries at some point in their life. About 0.4% of young women are affected in a given year and it is estimated to occur ten times more commonly among women than men. It is unclear whether the increased incidence of anorexia observed in the 20th and 21st centuries is due to an actual increase in its frequency or simply due to improved diagnostic capabilities. In 2013, it directly resulted in about 600 deaths globally, up from 400 deaths in 1990. Eating disorders also increase a person's risk of death from a wide range of other causes, including suicide. About 5% of people with anorexia die from complications over a ten-year period with medical complications and suicide being the primary and secondary causes of death respectively. Anorexia has one of the highest death rates among mental illnesses, second only to opioid overdoses.