This section is from the "A Handbook of Useful Drugs" book, by State Medical Examining and Licensing Boards.
The beetle Cantharis vesicatoria, Spanish fly, thoroughly dried.
Action and Uses : Cantharides is very irritating to the intestinal canal, producing hyperemia of the mouth and throat and vomiting. It is readily absorbed from the intestinal canal and produces marked irritation of the kidneys. The nephritis affects, at first, the glomeruli and subsequently the urinary tubules. In its passage through the urinary channels, it irritates the mucous membranes of the bladder and urethra and produces a desire to urinate, sometimes amounting to strangury. Cantharides is also a local irritant to the skin and produces blisters. It may be absorbed from the skin in sufficient quantities to cause nephritis. Cantharides has been used for treatment of chronic nephritis and incontinence of urine. Only minute doses should ever be given for these affections, and as soon as any evidence of vesical irritation arises the remedy should be suspended. Many physicians believe that cantharides should never be used internally.
The local irritant action of cantharides is the basis of its use for the treatment of baldness, but it is of little benefit in that condition. It is used for baldness in the form of tincture greatly diluted with alcohol (from 1:15 to 1:30) or in ointments. The chief use of cantharides is as a vesicant. It is contra-indicated in nephritis, and when vesication is desired in nephritis, another agent such as ammonia or chloroform should be selected. It may be used to produce redness and counterirritation in quantities not sufficient to blister. When the irritation is carried just to the point of beginning vesication, the result is known as a flying blister. The counterirritation may be rendered continuous by a succession of such "flying" blisters.