This section is from the book "Materia Medica Pharmacy, Pharmacology And Therapeutics", by W. Hale White. Also available from Amazon: Materia Medica Pharmacy, Pharmacology And Therapeutics..
Mustard is a typical powerful local irritant. Thus it first produces dilatation of the vessels, which causes redness of the skin (rubefacient effect) and a sensation of warmth. Because of the irritant action of mustard on the sensory nerves, a severe burning pain is soon felt. The irritation of the nerves is followed by their paralysis, consequently there is a local loss of sensibility, and a diminution both of the pain produced by the mustard and of any that may have been present before its application. The irritation of the vessels leads to the transudation of plasma through them; this, collecting under the epidermis, raises it, and thus vesicles, blebs, or blisters, are formed (vesicant effect.) Mustard is also a counter-irritant (see p. 61); that is to say, the stimulation of the cutaneous nerves reflexly leads to an alteration in the size of the vessels of the viscera under the seat of application.
This excitation of the sensory nerves is sufficiently powerful to reflexly stimulate the heart and respiration, and sometimes to restore consciousness after fainting.
Gastro-intestinal tract. - Mustard also acts here as an irritant. Taken in the usual small quantities as a condiment, it causes a sense of warmth in the stomach, it moderately stimulates the secretion of gastric juice and the peristaltic movements, and therefore sharpens the appetite. A dose of one to four teaspoonfuls 4. to 15. gm. stirred up in a tumbler of lukewarm water is sufficiently irritating to be a direct stomachic emetic, causing prompt vomiting without the depression which usually attends emetics, because the mustard reflexly stimulates the heart and respiration.
A poultice made with flaxseed and having a little mustard (1 to 16 of flaxseed) sprinkled on it is a very common and efficacious application as an irritant and counter-irritant in rheumatism, pleurisy, pneumonia, bronchitis, pericarditis, and many inflammatory diseases. In the manner already explained, it will, when applied to the skin, soothe pain in gastralgia, colic, painful diseases of the chest, neuralgia, lumbago, etc. The paper or any of the mustard leaves that are sold, moistened in water, form an excellent application. Often the local application of mustard over the stomach relieves vomiting. A large mustard poultice applied to the legs was formerly used as a reflex stimulant in cases of syncope, asphyxia, and coma.
Common colds and febrile conditions, especially in children, are often treated by placing the feet and legs or the whole body in mustard and water as hot as can be borne 1 to 128, the object being by the cutaneous dilatation to withdraw blood from the inflamed part. A mustard sitz-bath may be taken at the time of the expected period, to induce menstruation.
Mustard is used as a condiment, and also as an emetic. It is especially valuable for poisoning by narcotics, because of its reflex stimulant effects.