Digestive tract
Causes in the digestive tract

Gastritis (inflammation of the gastric wall)
Gastroenteritis
Gastroesophageal reflux disease
Celiac disease
Non-celiac gluten sensitivity
Pyloric stenosis (in babies, this typically causes a very forceful "projectile vomiting" and is an indication for urgent surgery)
Bowel obstruction
Overeating (stomach too full)
Acute abdomen and/or peritonitis
Ileus
Food allergies (often in conjunction with hives or swelling)
Cholecystitis, pancreatitis, appendicitis, hepatitis
Food poisoning
In children, it can be caused by an allergic reaction to cows milk proteins (Milk allergy or lactose intolerance)

Sensory system and brain
Causes in the sensory system:
Movement leading to motion sickness (which is caused by overstimulation of the labyrinthine canals of the ear)
Ménières disease
VertigoCauses in the brain:
Concussion
Cerebral hemorrhage
Migraine
Brain tumors, which can cause the chemoreceptors to malfunction
Benign intracranial hypertension and hydrocephalusMetabolic disturbances (these may irritate both the stomach and the parts of the brain that coordinate vomiting):
Hypercalcemia (high calcium levels)
Uremia (urea accumulation, usually due to kidney failure)
Adrenal insufficiency
Hypoglycemia
HyperglycemiaPregnancy:
Hyperemesis, morning sicknessDrug reaction (vomiting may occur as an acute somatic response to):
Alcohol, which can be partially oxidized into acetaldehyde that causes the symptoms of hangover, including nausea, vomiting, shortness of breath, and fast heart rate. Opioids
Selective serotonin reuptake inhibitors
Many chemotherapy drugs
Some entheogens (such as peyote or ayahuasca)Illness (sometimes colloquially known as "stomach flu"—a broad name that refers to gastric inflammation caused by a range of viruses and bacteria):
Norovirus (formerly Norwalk virus or Norwalk agent)
Swine influenzaPsychiatric/behavioral:

Bulimia nervosa
Purging disorder

Emetics
An emetic, such as syrup of ipecac, is a substance that induces vomiting when administered orally or by injection. An emetic is used medically when a substance has been ingested and must be expelled from the body immediately.
This phenomenon has been touched on in popular culture: notorious instances appear in the films Monty Pythons The Meaning of Life (1983) and Stand By Me (1986).Intense vomiting in ayahuasca ceremonies is a common phenomenon. However, people who experience "la purga" after drinking ayahuasca, in general, regard the practise as both a physical and spiritual cleanse and often come to welcome it. It has been suggested that the consistent emetic effects of ayahuasca—in addition to its many other therapeutic properties—was of medicinal benefit to indigenous peoples of the Amazon, in helping to clear parasites from the gastrointestinal system.There have also been documented cases of a single ill and vomiting individual inadvertently causing others to vomit, when they are especially fearful of also becoming ill, through a form of mass hysteria. Most people try to contain their vomit by vomiting into a sink, toilet, or trash can, as vomit is difficult and unpleasant to clean. On airplanes and boats, special bags are supplied for sick passengers to vomit into. A special disposable bag (leakproof, puncture-resistant, odorless) containing absorbent material that solidifies the vomit quickly is also available, making it convenient and safe to store until there is an opportunity to dispose of it conveniently.People who vomit chronically (e.g., as part of an eating disorder such as bulimia nervosa) may devise various ways to hide this disorder.An online study of peoples responses to "horrible sounds" found vomiting "the most disgusting".