Cantharidin Application Protocol
Primary Application Technique
Cantharidin is applied directly to wart or molluscum lesions as a painless, office-based blistering agent that triggers acantholysis, with the superficial nature of injury reducing scarring risk. 1
Preparation and Application Steps
Debride the lesion first by paring down excess keratin on warts to improve penetration before applying cantharidin 1
Apply cantharidin 0.7% solution directly to the lesion surface using a cotton-tipped applicator or similar device, taking care to apply only to the wart or molluscum and avoid surrounding normal skin 1, 2
Allow the solution to dry completely before the patient leaves the office, which produces a visible film on the treated area 2
Occlusion is optional - studies show cantharidin works both with and without occlusive dressing, though some practitioners prefer occlusion for 4-6 hours 3, 4
Instruct patients to wash off the treated area after 4-6 hours (or sooner if significant discomfort develops) to limit the blistering response 2, 4
Formulation Selection by Indication
For molluscum contagiosum and common warts: Use cantharidin 0.7% alone (Canthacur or Ycanth formulations) 2, 5
For plantar warts specifically: Use the more potent combination formulation containing 1% cantharidin, 30% salicylic acid, and 2% podophyllotoxin (Canthacur PS), which demonstrates 81-100% clearance rates, often after a single treatment 6, 7, 2
The British Association of Dermatologists guidelines note that cantharidin 0.7% achieved clearance in all 15 patients with plane facial warts within 16 weeks using one to four treatments 1
Treatment Schedule and Follow-Up
Repeat applications weekly if lesions persist, as most patients require 1-4 treatment sessions 1, 4
For molluscum contagiosum: Clearance rates range from 15.4-100% depending on the study, with 36.2% achieving complete clearance versus 10.6% with placebo in the highest quality randomized trial 7, 3
For warts: Significant clearance is maintained with cantharidin alone, particularly when combined with podophyllotoxin and salicylic acid for plantar warts 7
Identify and treat all lesions including nascent ones during each visit to reduce recurrence risk, as reducing viral load allows the host immune response to eliminate residual virus 8, 9
Expected Adverse Effects and Management
Pain and blistering are the most common side effects, occurring in 7-85.7% and 10-100% of patients respectively, but these are transient and expected as part of the mechanism of action 7, 3
Discomfort develops 24 hours after application when blistering occurs, not during the painless application itself 1
Hyperpigmentation or hypopigmentation occurs in 1.8-53.3% of patients, particularly in darker skin types 7
No serious adverse events were observed in controlled trials, and patient satisfaction is high, especially for molluscum contagiosum 7, 3
Critical Safety Warnings
Cantharidin is highly toxic if taken systemically and causes gastrointestinal and genitourinary tract ulceration, electrolyte disturbances, and renal dysfunction 2
Store securely away from children and ensure patients understand it is for external use only 2
Apply sparingly and only to lesions to minimize systemic absorption risk 2
Cantharidin is not FDA-approved, though it has been used safely in dermatology since the 1950s 2, 4
Common Pitfalls to Avoid
Do not apply cantharidin to surrounding normal skin, as this causes unnecessary blistering and pain - precise application is essential 2, 4
Do not use the potent Canthacur PS formulation (with salicylic acid and podophyllotoxin) for facial, periungual, or genital lesions - reserve this only for plantar warts 2, 5
Warn patients about expected blistering so they understand this is the intended mechanism and not a complication 1, 4
For periocular molluscum with conjunctivitis, physical removal (curettage, excision, or cryotherapy) is preferred over cantharidin to ensure rapid resolution of eye symptoms 8, 9