Blisters are medicinal agents which, when applied to the skin, raise the cuticle into vesicles filled with serous fluid. They are applied either in the form of plasters or in a fluid state, as suits the convenience of the person or part, and have for their object the establishing of a counter-irritation or diversion of inflammatory action from a part in which it cannot be reached by remedies, or from some organ where it may do permanent mischief, to some more superficial part of the body.
The most common blister in use is made of Cantharides or Spanish fly (Cantharis vesicatoria). Cantharides, mixed with a convenient proportion of lard and wax, form the blistering ointment in ordinary use; the only objection to this preparation being, that if applied too freely, it produces much irritation of the kidneys and urinary bladder. In young children and very thin-skinned persons, a layer of silver paper, or thin gauze wet with vinegar, may be laid between the blister and the skin. But under no circumstances should a blister be left long upon children, as it may produce sores which are apt to take on an unhealthy action, and are difficult to heal.
Mustard (Sinapis nigra) is frequently used to redden the skin, but seldom left on sufficiently long to produce blistering. Tincture of cantharides, croton-oil, strong liquor ammoniae, tartar emetic ointment, and many other drugs are used in practice.
If the occasion for the blister passes off, the vesicles should be pricked, and their fluid contents allowed to trickle away, the vesicated surface being then dressed with some cold cream or lard. But if it should appear desirable to promote a discharge from the skin, the raised cuticle may be snipped off, and some stimulating application such as savine ointment (Juniperus sabina) used to dress the raw surface. Great cleanliness should be observed in dressing the part.
Blisters are used for the relief of acute inflammation in internal organs—e.g. the pleura, lungs, membranes of the brain; for the dispersion of the products left by inflammation—e.g. in indolent ulcers of the skin, enlarged lymphatic glands, and chronically inflamed joints; and for the relief of pain, even where no inflammation is present—e.g. in many forms of neuralgia. To name all the conditions where blisters may be of use would mean an enumeration of more than half the diseases known to medicine and surgery.
Blister is also one of the names applied to a bleb on the skin filled with fluid, by whatever cause produced—friction, disease, &c., as well as by vesicants, or other medicinal agents. See BULLA.