What causes hiccups, and does anything actually stop them?

A hiccup is a sudden involuntary contraction of the diaphragm, followed about 35 milliseconds later by abrupt closure of the vocal cords, which produces the characteristic sound. The reflex arc involves the phrenic and vagus nerves and a control center in the brainstem. Common triggers include eating too quickly, swallowing air, carbonated drinks, alcohol, sudden temperature changes in the stomach, and excitement or stress, all of which irritate or stimulate the nerves involved.

Why the reflex exists at all is unresolved, and the most interesting hypothesis is evolutionary. The pattern resembles the gill ventilation motion of amphibian ancestors, and it also closely resembles the suckling reflex in infants, where a diaphragm contraction with a closed glottis would help clear air from the stomach during nursing. Fetuses hiccup extensively in utero, well before breathing, which supports the idea that the circuit serves some developmental purpose.

The folk remedies that work do so through two mechanisms, which is why the successful ones cluster. Raising blood carbon dioxide, by holding your breath or breathing into a paper bag, appears to suppress the reflex. Stimulating the vagus nerve does the same, which explains swallowing granulated sugar, drinking cold water quickly, gargling, pulling on the tongue, and the Valsalva maneuver. Neither is guaranteed, and the placebo component is significant. What warrants medical attention is persistent hiccups lasting more than 48 hours, which are uncommon and can signal underlying conditions including nerve irritation, gastroesophageal reflux, metabolic disturbance, or central nervous system problems. The longest documented case lasted 68 years.