These are drugs which excite more or less local irritation when applied to the skin; the inflammatory condition is accompanied by an effusion of serum between the epidermis and dermis - i. e., a blister.

The principal vesicants are:

Acetic acid (glacial), Ammonia (the confined vapor), Cantharides, Iodine, Mezereon, Mustard (volatile oil), Rhus Toxicodendron.

There are certain drugs which affect certain parts of the skin - for instance, the orifices of the sudoriferous glands - in a special manner, and their action on these parts is such as to give rise to pustules rather than blisters. Drugs which affect the skin in this manner are called Pustulants. The following-named drugs are the most important of them:

Croton oil, Tartar emetic, Silver nitrate, Ipecac.

Therapeutics. - Vesicants are employed as local stimulants in chronic ulcers and to facilitate the absorption of effusions, as in chronic synovitis or chronic thickening about the joints.

Blisters are also of use in endocarditis, neuralgias, sciatica, chronic pericarditis, pleurisy, hysterical paralysis, and aphonia, cerebral or spinal meningitis, etc.

Pustulants are more particularly employed to maintain a continuous though moderate irritation in chronic inflammations. They are but rarely used for the same class of cases as vesicants, but are preferable when it is desirable to prolong the local irritation without exciting too much inflammation.

Contraindications. - Vesicants are usually contraindicated in acute inflammations and in inflammation of the cutaneous tissues, as rubeola and scarlatina. Vesicants are not permissible in pregnancy, debility, scorbutus, and purpura, or in extreme infancy and old age. They should not be applied over the scrotum or the mammary glands, nor over bony prominences where the healing processes are apt to be retarded.