Vomiting is the forceful ejection of food and fluid from the stomach and proximal duodenum. Vomitus containing undigested food suggests gastric origin, while digested food containing bile suggests intestinal origin. Regurgitation is the passive expulsion of food and fluid from the oral cavity, pharyngeal cavity, or the esophagus. Regurgitated food is undigested. Regurgitated food in a tubular shape suggests esophageal origin.

The vomiting reflex begins with stimulation of afferent receptors located in visceral organs (i.e., gastrointestinal tract, peritoneum, pancreas, liver, kidneys, urinary bladder, and reproductive tract) by acute distention, irritation of the small intestinal mucosa, hypertonicity, acute erosions/ulceration, and chemical irritation. The duodenum contains the highest number of afferent receptors. After stimulation, these afferent impulses travel through the vagus and sympathetic nerves to the vomiting center located in the medulla. In addition to the peripheral visceral receptors, the vomiting center receives input from the chemoreceptor trigger zone located on the floor of the fourth ventricle, from the vestibular system, and from the higher centers of the brain. The chemoreceptor trigger zone mediates vomiting secondary to drugs and toxins, such as apomorphine, cardiac glycosides, and uremia. The vestibular system is responsible for vomiting associated with either vestibular disease or motion sickness.

Vomiting is divided into three phases: nausea, retching, and vomiting. During the nausea phase, the animal will have increased salivation and subsequent increase in swallowing frequency. This serves to coat and protect the esophagus from the acidic gastric contents. Other signs include lethargy, shivering, hiding, yawning, and licking of the lips. There is a decrease in gastric and esophageal motility, along with relaxation of the lower esophageal sphincter. Retrograde motility of the proximal small intestines moves food contents (and bile) back into the stomach. The second phase, retching, consists of forceful abdominal contractions. These contractions create positive intra-abdominal pressure and a negative intrathoracic pressure, which serve to move food toward the esophagus. The process is completed by forcefully expelling the vomitus out of the mouth. Contraction of abdominal muscles and the diaphragm provides the driving force needed for this to occur. The nasopharynx and glottis close as the vomitus traverses the pharynx minimizing aspiration into the airways.

At times, differentiating regurgitation from vomiting is difficult. Careful history taking, description of the material expelled (i.e., consistency, presence of bile or blood, volume, and odor), and the relationship of the episodes to eating or drinking should be determined. Because vomiting involves a forceful contraction of the abdominal muscles and regurgitation is passive, the owner’s description of the episode can be helpful. Also, if the vomiting occurs 8–10 hours after eating (long after the stomach should have emptied), then hypomotility or gastric outflow obstruction should be suspected. Occasionally, paroxysmal coughing or gagging can be followed by regurgitation or vomiting, making the situation more confusing. In addition, coughing can be confused for vomiting if followed by the expectoration of excessive mucus. Although the above-mentioned signs can be helpful, they do not always distinguish vomiting from regurgitation. Potential complications of vomiting include aspiration pneumonia, malnutrition, electrolyte imbalances, acid–base disturbances, and dehydration. Potential complications of regurgitation include malnutrition, aspiration pneumonia, and dehydration.

The initial diagnostic plan for vomiting animals is to distinguish primary gastrointestinal causes from non-gastrointestinal or metabolic causes. This is typically accomplished through abdominal imaging (abdominal radiographs, abdominal ultrasound, and ± contrast radiography) and a standard small animal profile (hemogram, serum biochemical panel, and urinalysis). Thoracic radiographs should be evaluated in all cases of regurgitation and in cases of vomiting when aspiration pneumonia is suspected. For patients with severe vomiting and concurrent polysystemic clinical signs, an extended database may be needed to establish an etiologic diagnosis. Special laboratory tests such as pancreatic lipase immunoreactivity (pancreatitis), bile acids (liver failure), adrenocorticotropic stimulation test or ACTH stimulation (hypoadrenocorticism), fecal parvovirus antigen assay, and abdominocentesis (peritoneal effusion) may be indicated.