Cantharidine is a substance derived from the blister beetle Cantharis vesicatoria. The Chinese have used this ancient medicine for thousands of years for a number of maladies. In the 1950's it was used in the US and other westernised countries to treat warts. However, in 1962 due to new manufacturing regulations it lost its FDA approval and was removed from the market. This saw a decline in its use until recently where it is being re-evaluated as a wart remover that doesn't cause scarring.
Cantharidine is also known as Spanish Fly and its beetle juice sold as an aphrodisiac. The reality is that it is ineffective for this purpose and in fact if swallowed is poisonous and possibly even fatal.
How does cantharidine work and what is it used for?
Cantharidine is a vesicant that causes a blister to form on the wart or growth. This action lifts the wart off the skin and after a few days when the blister has dried the wart will come off. The action of cantharidine does not go beyond the epidermal cells, the basal layer remains intact hence no scarring. Cantharidine is sometimes effective in treating common viral warts and very frequently effective for molluscum contagiosum. Both are viral skin infections that result in small, harmless skin growths.
How to use cantharidine
Because of the toxic potential of cantharidine it should only be used topically and in a professional office setting, applied to the lesions by a doctor.
Cantharidine should be used as follows.
- First the doctor pares or shaves the wart (this is not necessary for molluscum).
- Cantharidine (formulated with substances that create an oily or colloidal film) is accurately applied to the wart or molluscum.
- The liquid is allowed to dry and then covered and sealed with nonporous tape for 4-6 hours.
- The tape should then be removed and the area washed with soap and water.
- Within 24 to 48 hours a blister will form.
- Over the next few days the blister will dry and the lesion may fall off. If necessary the blistered lesion can be snipped off near the base (local anaesthetic may be required).
- Healing is normally complete within 4-7 days.
- Resistant warts or new molluscum lesions may require a repeat treatment.
Cantharidine should not be used on the following:
- Moles, birthmarks, or undiagnosed skin lesions.
- Warts or molluscum on oral mucous membranes (eyelids, nostrils, mouth) and should be used with care around the genitals and anus.
It should be used with caution in people with diabetes, peripheral vascular disease or other circulatory problems, as they are more likely to develop complications.
Usually the application of cantharidine on the wart does not hurt but the resulting blister can sometimes be uncomfortable. In a small number of patients, a ring of small satellite warts surrounding the original wart may appear after cantharidine treatment. However, this complication can just as likely occur with other wart removal therapies.
There is the possibility of complications occurring if used to treat plantar warts on the soles of the feet. Isolated reports of inflammation of lymph vessels and cellulitis have been documented.