The dried and powdered bodies of the Cantharis vesicatoria, a beetle of Southern Europe. The powder is grayish-brown and specked with minute greenish spangles from the wing-cases of heads. It has a strong unpleasant odor. The active principle is cantharidin, an active irritant, besides which it contains a volatile oil and fatty substances.

Physiological Actions

Externally cantharides is vesicant. When applied to the skin a feeling of heat and burning is felt in a few hours, and small vesicles form which unite in one large blister.

The average time required for this result is about eight hours. The action of cantharides is attended with less injury to the skin than that of any other vesicant. No pus is formed during the healing process, and no scar is left by the blister.

Internally cantharides is irritant to mucous membrane, and if given medicinally must be largely diluted. It enters the blood from the stomach and also from blistered surfaces, and is slowly excreted by the kidneys. In small doses it causes diuresis with some irritation of the urinary organs, and larger doses produce strangury.

Symptoms Of Poisoning

When a poisonous dose of cantharides is taken, the first symptoms are burning in the oesophagus and stomach, a constricted feeling about the throat, gastric and abdominal pain, with vomiting and in most cases diarrhoea.

If the powder has been taken, the small green specks may be seen in the matter vomited, which is at first mucous, then bilious, and finally serous. The discharges from the bowels have the same characteristics and are scanty, frequent, and accompanied by tenesmus. There is frequently salivation with swelling of the salivary glands. The pulse is weak and rapid, and death usually occurs quickly from collapse caused by the gastro - intestinal inflammation, but if it is delayed for a few hours the symptoms of irritation of the urinary apparatus appear, beginning with pains in the back, and ending in strangury, with scanty, albuminous, or bloody urine, and tenesmus of the bladder.

Treatment Of Poisoning

There is no antidote to cantharides, and the stomach must be at once emptied, and as thoroughly as possible washed out; large quantities of albuminous and mucilaginous drinks given; warm baths to relieve the strangury, and stimulants if necessary. No oils or glycerin must be given, as they aid in the absorption of the poison.

It is to be remembered that constitutional effects are sometimes produced by even a moderate blister, and if necessary the blister must be removed and the part washed with soap and water.


Before applying a blister the spot should be washed with soap and water; dried; washed again with alcohol or ether, and briskly rubbed for a moment or two. Absorption then takes place more quickly. A blister should never be applied over a bony prominence, as sloughing may follow, the circulation in such parts being sluggish. On tender skins vesication is soon produced and the blister must be carefully watched lest the action be too severe. On coarse skins, or in places where it is thick, as on the scalp or at the knee-joint, more time is needed.

Hairs must be cut away, or shaved. A blister should not be left on a child's skin long enough to rise, but should be removed when redness appears, and poultices be applied to finish the process.