This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Soothing remedies applied to the pharynx greatly relieve cough, although they do not reach so far down as the epiglottis. Mucilaginous remedies are very useful for this purpose, and they may either be employed alone or as vehicles for the local application of sedatives such as morphine. Thus, a piece of extract of liquorice allowed to dissolve in the mouth, a marsh-mallow lozenge, a gum-jujube, or a sip of linseed-tea, by covering the back of the throat with a mucilaginous coating, will lessen cough to a great extent. Such remedies are especially useful where the cough depends on congestion of the pharynx and trachea. In such cases no abnormal sound at all may be heard in auscultation, and the cough being due to irritation of the parts supplied by the superior laryngeal nerve, has a peculiarly convulsive expiratory character often termed 'barking.'
Other remedies lessen cough by diminishing congestion of the respiratory passages, and thus lessening the irritation which causes the cough. Many of these also, however, come under the class of expectorants (p. 250), inasmuch as the diminished congestion is frequently associated with increase of the expectoration. Others, again, although they diminish cough, are included rather under the head of 'cardiac tonics,' or sedatives. Digitalis is an example of this. In the congestion due to cardiac disease, and even in that due to bronchitis, digitalis, by strengthening the heart and by contracting the vessels, may lessen the congestion in the lungs, and give the patient relief. Squill and a number of other drugs have an action on the bloodvessels similar to that of digitalis.
Other remedies, such as the vapour of hydrocyanic acid, conium, stramonium, and tobacco, have a local sedative action on the lung, and may lessen cough; they also are used in order to diminish local spasm of the bronchioles, and thus to relieve spasmodic asthma.