Anorexia is a medical term for a loss of appetite. While the term in non-scientific publications is often used interchangeably with anorexia nervosa, many possible causes exist for a loss of appetite, some of which may be harmless, while others indicate a serious clinical condition or pose a significant risk.

Anorexia is a symptom, not a diagnosis. Anorexia is not to be confused with the mental health disorder anorexia nervosa. Because the term 'anorexia' is often used as a short-form of anorexia nervosa, to avoid confusion a provider must clarify to a patient whether they are simply referring to a decreased appetite or the mental health disorder. Anyone can manifest anorexia as a loss of appetite, regardless of their gender, age, or weight.

The symptom also occurs in other animals, such as cats, dogs, cattle, goats, and sheep.In these species, anorexia may be referred to as inappetence. As in humans, loss of appetite can be due to a range of diseases and conditions, as well as environmental and psychological factors.

Appetite stimulation and suppression is a complex process involving many different parts of the brain and body by the use of various hormones and signals. Appetite is thought to be stimulated by interplay between peripheral signals to the brain (taste, smell, sight, gut hormones) as well as the balance of neurotransmitters and neuropeptides in the hypothalamus.[8] Examples of these signals or hormones include neuropeptide Y, leptin, ghrelin, insulin, serotonin, and orexins (also called hypocretins). Anything that causes an imbalance of these signals or hormones can lead to the symptom of anorexia. While it is known that these signals and hormones help control appetite, the complicated mechanisms regarding a pathological increase or decrease in appetite are still being explored.[8]

• Acute radiation syndrome
• Addison's disease
• Alcohol withdrawal
• Anemia
• Anorexia nervosa
• Anxiety
• Appendicitis
• Babesiosis
• Benzodiazepine withdrawal
• Bipolar disorder
• Cancer[9][10]
• Cannabinoid hyperemesis syndrome
• Cannabis withdrawal
• Celiac disease[11]
• Chronic kidney disease
• Chronic pain
• Common cold
• Constipation
• COPD
• COVID-19
• Crohn's disease
• Dehydration
• Dementia
• Depression
• Drug side effect
• Ebola
• Fatty liver disease
• Fever
• Food poisoning
• Gastroparesis
• Hepatitis
• HIV/AIDS
• Hypercalcemia
• Hyperglycemia
• Hypervitaminosis D
• Hypothyroidism and sometimes hyperthyroidism
• Irritable bowel syndrome
• Ketoacidosis
• Kidney failure[12]
• Mania
• Metabolic disorders, particularly urea cycle disorders
• MELAS syndrome
• Nausea
• Opioid use disorder
• Pancreatitis
• Psychosis
• Schizophrenia
• Stimulant use disorder
• Stomach flu
• Stress
• Sickness behavior[13][14]
• Superior mesenteric artery syndrome
• Tuberculosis
• Thalassemia
• Ulcerative colitis
• Vitamin B12 deficiency
• Zinc deficiency

Infection : Anorexia of infection is part of the acute phase response (APR) to infection. The APR can be triggered by lipopolysaccharides and peptidoglycans from bacterial cell walls, bacterial DNA, and double-stranded viral RNA, and viral glycoproteins, which can trigger production of a variety of proinflammatory cytokines. These can have an indirect effect on appetite by a number of means, including peripheral afferents from their sites of production in the body, by enhancing production of leptin from fat stores. Inflammatory cytokines can also signal to the central nervous system more directly by specialized transport mechanisms through the blood–brain barrier, via circumventricular organs (which are outside the barrier), or by triggering production of eicosanoids in the endothelial cells of the brain vasculature. Ultimately, the control of appetite by this mechanism is thought to be mediated by the same factors normally controlling appetite, such as neurotransmitters (serotonin, dopamine, histamine, norepinephrine, corticotropin releasing factor, neuropeptide Y, and α-melanocyte-stimulating hormone).

Stimulants, such as ephedrine, amphetamine, methamphetamine, MDMA, cathinone, methylphenidate, nicotine, cocaine, caffeine, etc. Narcotics, such as heroin, morphine, codeine, hydrocodone, oxycodone, etc. Antidepressants can have anorexia as a side effect, primarily selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine. Byetta, a type II diabetes drug, will cause moderate nausea and loss of appetite. Abruptly stopping appetite-increasing drugs, such as cannabis and corticosteroids. Chemicals that are members of the phenethylamine group. (Individuals with anorexia nervosa may seek them to suppress appetite.) Topiramate may cause anorexia as a side effect. Other drugs may be used to intentionally cause anorexia in order to help a patient preoperative fasting prior to general anesthesia. It is important to avoid food before surgery to mitigate the risk of pulmonary aspiration, which can be fatal.

During the post-operative recovery period for a tonsillectomy or adenoidectomy, it is common for adult patients to experience a lack of appetite until their throat significantly heals (usually 10–14 days).[16] Altitude sickness Significant emotional pain caused by an event (rather than a mental disorder) can cause an individual to temporarily lose all interest in food. Psychological stress Experiencing grotesque or unappealing thoughts or conversations, or viewing similar images Being in the presence of unappealing things such as waste matter, dead organisms, or bad smells

Complications of anorexia may result due to poor food intake. Poor food intake can lead to dehydration, electrolyte imbalances, anemia and nutritional deficiencies. These imbalances will worsen the longer that food is avoided.

Anorexia is a relatively common condition that can lead patients to have dangerous electrolyte imbalances, leading to acquired long QT syndrome which can result in sudden cardiac death. This can develop over a prolonged period of time, and the risk is further heightened when feeding resumes after a period of abstaining from consumption.

Care must be taken when a patient begins to eat after prolonged starvation to avoid the potentially fatal complications of refeeding syndrome. The initial signs of refeeding syndrome are minimal, but can rapidly progress to death. Thus, the reinitiation of food or oral intake is usually started slowly and requires close observation under supervision by trained healthcare professionals. This is usually done in a hospital or nutritional rehabilitation center.