Hiccups affect people of all ages, even being observed in utero. They become less frequent with advancing age. Intractable hiccups, lasting more than a month, are more common in adults. While males and females are affected equally often, men are more likely to develop protracted and intractable hiccups.

Along with humans, hiccups have been studied and observed in cats, rats, rabbits, dogs, and horses.

• Food stuck in the esophagus
• Swallowing air excessively
• Gastroesophageal reflux
• Hiatal hernia
• Rapid eating
• Alcohol or carbonated beverages
• Spicy foods
• Opiate drug use
• Laughing vigorously or for a long time
Hiccups may be triggered by a number of common human conditions. In rare cases, they can be a sign of serious medical problems such as myocardial infarction.
• Kidney failure
• Stroke
• Multiple sclerosis
• Meningitis
• Damage to the vagus nerve after surgery
• Although no clear pathophysiological mechanism has been described, hiccups is known to have been the initial symptom of Plasmodium vivax malaria in at least one documented case.
A leading hypothesis is that hiccups evolved to facilitate greater milk consumption in young mammals. The coordination of breathing and swallowing during suckling is a complicated process. Some air inevitably enters the stomach, occupying space that could otherwise be optimally used for calorie-rich milk.

The hypothesis suggests that the presence of an air bubble in the stomach stimulates the sensory (afferent) limb of the reflex through receptors in the stomach, esophagus and along the underside of the diaphragm. This triggers the active part of the hiccup (efferent limb), sharply contracting the muscles of breathing and relaxing the muscles of the esophagus, then closing the vocal cords to prevent air from entering the lungs. This creates suction in the chest, pulling air from the stomach up into the esophagus. As the respiratory muscles relax the air is expelled through the mouth, effectively "burping" the animal.

There are a number of characteristics of hiccups that support this theory. The burping of a suckling infant may increase its capacity for milk by more than 15–25%, bringing a significant survival advantage. There is a strong tendency for infants to get hiccups, and although the reflex persists throughout life it decreases in frequency with age. The location of the sensory nerves that trigger the reflex suggests it is a response to a condition in the stomach. The component of the reflex that suppresses peristalsis in the esophagus while the airway is being actively blocked suggests the esophagus is involved. Additionally, hiccups are only described in mammals, the group of animals that share the trait of suckling their young.
An international respiratory research group composed of members from Canada, France, and Japan proposed that the hiccup is an evolutionary remnant of earlier amphibian respiration. Amphibians such as tadpoles gulp air …

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Episodes of hiccups usually last under 30 minutes. Prolonged attacks, while rare, can be serious. Root causes of prolonged hiccups episodes are difficult to diagnose. Such attacks can cause significant morbidity and even death. An episode lasting more than a few minutes is termed a bout; a bout of over 48 hours is termed persistent or protracted. Hiccups lasting longer than a month are termed intractable. In many cases, only a single hemidiaphragm, usually the left one, is affected, although both may be involved.