Cantharidin is a blister-inducing medication derived from the blister beetle that is applied directly to the wart by a clinician. It causes the wart tissue to lift and separate from the healthy skin underneath, usually forming a blister within 24 hours that leads the wart to slough off over the next several days. The treatment itself is typically painless at the time of application, making it useful for children or those sensitive to cryotherapy. Multiple sessions may be needed for complete clearance, especially for plantar warts. Because of the risk of excessive blistering if misused, cantharidin is not sold over the counter and must be applied in a controlled medical setting.