Although sometimes unsettling and a bit scary, vomiting up mucus is fairly common. Rather than an actual illness, vomiting mucus is actually an effect that can be caused by a variety of factors. It may be caused by a severe cold, an upper respiratory infection, allergies, or a bad cough. Acid reflux may also be a cause.

Mucus is a fluid secreted by mucus membranes. The human body produces anywhere from a quart to a gallon (0.94 to 3.78 liters) of mucus a day. Mucus coats the walls of the nasal passages to collect particles, such as dust or pollen, that might irritate someone and cause him or her to sneeze. It also lubricates the air passages, making it easier to breathe, and coats the esophagus and stomach to protect these organs from stomach acid.

Sometimes, however, the body creates too much mucus, such as during a cold, an upper respiratory infection, an allergy attack, or a coughing fit. In these instances, the mucus leaks from the sinuses and runs down the back of the throat — called post nasal drip — or is coughed up from the lungs; it may then be swallowed and end up in the stomach. When a person swallows too much of this secretion, it can cause the body to begin vomiting mucus. Too much mucus or mucus that is too thick can cause nausea, and one of the body's natural responses is to trigger vomiting to ease that nausea.

In young children, one of the major reasons for vomiting mucus is due to their sensitive gag reflex. A child with a cold, allergies, or lung infection will secrete a great deal of mucus; that child may cough so forcefully trying to clear his or her airway that it triggers the gag reflex, and he or she vomits. In addition, children tend to swallow mucus rather than spitting it out or "coughing it clear" as adults do. This may occur when children have a severe infection, such as a sinus infection that creates thick, excessive secretions of mucus.

Another cause of vomiting mucus may be due to acid reflux or heartburn. Mucus coats the esophagus and stomach walls to prevent stomach acid from damaging or dissolving those organs. When one has acid reflux, the stomach acid backs up into the esophagus. In order to protect itself, the body produces more mucus secretions. This excessive secretion is often swallowed back down into the stomach, and when there is too much, a person might feel ill and vomit up the excess mucus.