Watkins P: Identifying and treating plantar warts. Nurs Stand 20: 50, 2006.
Kuhn D: Foot rings in a 10-year-old girl. Contemp Pediatr 30: 38, 2013.
Dormer Laboratories Inc: Cantharone Plus. Available at: http://www.dormer.com/Cantharone/AccDetail.aspx?ID=9002-975M. Accessed 2015. Accessed May 15, 2016.
Moed L, Shwayder TA, Chang MW: Cantharidin revisited: a blistering defense of an ancient medicine. Arch Dermatol 137: 14, 2001.
Alzoubi K, Egler J, Briglia M, et al: Introduction of suicidal erythrocyte death by cantharidin. Toxins (Basel) 7: 2822, 2015.
Torbeck R, Pan M, de Moll E, et al: Cantharidin: a comprehensive review of the clinical literature. Dermatol Online J 20: 1, 2014.
Kollipara R, Ekhlassi E, Downing C, et al: Advancements in pharmacotherapy for noncancerous manifestations of HPV. J Clin Med 4: 832, 2015.
López López D, Vilar Fernandez JM, Losa Iglesias ME, et al: Safety and effectiveness of cantharidin-podophylotoxin-salicylic acid in the treatment of recalcitrant plantar warts. Dermatol Ther 29: 269, 2016.
Epstein JH, Epstein WL: Cantharidin treatment of digital and periungual warts. Calif Med 93: 11, 1960.
Rosenberg EW: Cantharidin treatment of warts at home. Arch Dermatol 113: 1134, 1960.
Smolinski KN, Yan AC: How and when to treat molluscum contagiosum and warts in children. Pediatr Ann 34: 211, 2005.
Findlay GH: Wart relapses at the edges of therapeutic cantharidin blisters. Arch Dermatol 80: 589, 1959.
Lipke MM: An armamentarium of wart treatments. Clin Med Res 4: 273, 2006.
James WD, Berger T, Elston D: “Viral Diseases,” in Andrews' Diseases of the Skin: Clinical Dermatology, 12th Ed, p 402, Elsevier, Philadelphia, PA, 2016.
Panzer HM. Cantharidin: a useful agent in the local treatment of warts. J Germantown Hosp 2: 82, 1961.
Gibbs RC. An annular verruca. Arch Dermatol 91: 1, 1965.
Akdemir O, Bilkay U, Tiftikcioglu YO, et al: New alternative in treatment of callus. J Dermatol 38: 146, 2011.
Sagi L, Trau H: The Koebner phenomenon. Clin Dermatol 29: 231, 2011.
Verrucae (warts) are the most common viral infections of the skin, affecting 7% to 10% of the general population. Typically caused by human papillomavirus type 1, plantar warts manifest as benign proliferation of the epithelial cells on the feet. It has been cited that up to one-third of nongenital warts become recalcitrant, and biopsy is often required to confirm diagnosis and direct appropriate treatment. These treatments can vary from various types of oral medications, acids, ablative modalities, and injections. In this article, we present a case of a recalcitrant plantar wart that appeared to circumferentially spread from the initial site after first-line treatment and presumed resolution with the product cantharidin. The development of ring warts is a known complication associated with cantharidin use, with little described rationale to the presentation.