Counter-irritants, agents applied to the skin so as to redden (rubefacients), to vesicate (vesicatories or Blisters, q.v.), or to produce pustules, purulent issues, or even sloughs of skin and of the subcutaneous textures. The milder counter-irritants are mustard (see CATAPLASM), turpentine applied on warm cloths, and spirit or acetic acid in lotion. The stronger are blisters of Cantharides (q.v.) or of ammonia; Croton-oil (q.v.) or Tartar Emetic (q.v.), in ointment; setons, caustic or pea-issues, and the moxa; and above all, the actual Cautery (q.v.) or hot iron. None of the stronger counter-irritants should be used without careful consideration and medical advice; great mischief is often done by their careless or improper use. Counter-irritants relieve internal pain, check inflammations, and tend to promote the absorption of morbid effusions. Their effect is probably due mainly to reflex action, produced by the impression they cause upon the nerves of the skin to which they are applied.
Amongst horses, counter-irritants are much used for strains and diseases of the joints, but should never be applied, as they too often are, in recent cases, or whilst the part is hot or inflamed. Cantharidine preparations, or ointment of biniodide of mercury, are the most convenient. For cows, use fomentations, followed by the smart infraction of mustard-paste; for dogs, soap-liniment, strengthened, if required, by ammonia or turpentine.