In medicine, a cough (pronunciation Latin: tussis) is a sudden and often repetitively occurring defence reflex which helps to clear the large breathing passages from excess secretions, irritants, foreign particles and microbes. The cough reflex consists of three phases: an inhalation, a forced exhalation against a closed glottis (the complex of the vocal folds), and a violent release of air from the lungs following opening of the glottis, usually accompanied by a distinctive sound. Coughing can happen voluntarily as well as involuntarily.

Frequent or severe coughing usually indicates the presence of a disease. Many viruses and bacteria benefit by causing their host to cough, which helps to spread the disease to new hosts. Coughing is classified as acute (of sudden onset) if it is present less than three weeks, subacute if it is present between three and eight weeks, and chronic when lasting longer than eight weeks. A cough can be dry or productive, depending on whether sputum is coughed up. Most of the times, coughing is acute and caused by a respiratory tract infection. Coughing can be triggered by food entering the windpipe rather than the esophagus due to a failure of the epiglottis in patients who have difficulties swallowing. Smoking and air pollution are common causes of coughing. Provided the patient is a non-smoker and has a normal chest X-ray, the three most common causes of chronic cough are asthma, gastroesophageal reflux disease and post-nasal drip. Other causes of chronic cough include chronic bronchitis, heart failure and medications such as ACE inhibitors.

Since cough is a natural protective reflex, suppressing the cough reflex might have deleterious effects, especially if the cough is productive. Nonetheless, coughing might be severe enough (in terms of psychological, physical and social distress) to warrant treatment. This should be targeted towards the cause as much as possible, for example by smoking cessation and discontinuing ACE inhibitors. Some patients may only be worried about serious illnesses, and reassurance may suffice. Cough suppressants (or antitussives) such as codeine or dextromethorphan are frequently prescribed although scientific evidence supporting their use is often of poor quality. Other treatment options may target airway inflammation or may promote mucus expectoration.

Physiology

A cough is a protective, primitive reflex in healthy individuals. The cough reflex is initiated by stimulation of two different classes of afferent nerves, namely the myelinated rapidly adapting receptors, and nonmyelinated C-fibers with endings in the lungs. However it is not certain that the stimulation of nonmyelinated C-fibers leads to cough with a reflex as it's meant in physiology (with its own five components): this stimulation may cause mast cells degranulation (through an asso-assonic reflex) and edema which may work as a stimulus for rapidly adapting receptors.