Cantharides is used for this purpose, either as a plaster or in liquid
form; that is, in the form of vesicating collodion. Its most frequent
use is as a plaster, rarely over one and a half inches square. Scrub
the parts well with soap and water, and dry and rub with a soft cloth
until the skin is red, then apply the plaster and bandage lightly,
so there may be plenty of room for the formation of the blister.
Vesication is produced generally within four to eight hours; if it
is not, the plaster should be removed and the operation completed by
applying a poultice. Great care should be exercised when blistering
patients suffering from kidney affections. Cantharides is an acute
irritant, and not infrequently causes strangury or acute nephritis.
When the cantharidal collodion is used the space to be covered is
outlined with oil to prevent spreading; the collodion is applied with
a camel’s hair brush or other applicator, and covered with a layer of
soft lint and oil silk.
_Dressing the Blister._—Puncture the lower part of the bleb; absorb the
fluid with a piece of soft cotton; anoint with pure vaselin, and apply
a soft dressing. Bandage lightly.
Sometimes it is desirable to have the fluid reabsorbed into the system,
in which case precautions must be taken to prevent the blister from
rupturing.