What is the proper protocol for using cantharone (cantharidin) to treat skin conditions like warts or molluscum contagiosum?

How to Use Cantharidin (Cantharone)

Cantharidin 0.7% topical solution should be applied directly to warts or molluscum contagiosum lesions by a healthcare provider, left on for 4-6 hours (or less for sensitive areas), then washed off thoroughly by the patient at home, with repeat applications every 2-4 weeks as needed until lesions clear. 1, 2

Indications and Formulations

Cantharidin is available in different concentrations for specific conditions 1:

• 0.7% cantharidin (Canthacur, YCANTH): For common warts, periungual warts, and molluscum contagiosum 1, 3
• 1% cantharidin + 30% salicylic acid + 2% podophyllotoxin (Canthacur PS): For plantar warts only (more potent formulation) 1, 4

Application Protocol

Pre-Application Preparation

• Debride or pare thick keratin from warts before application to enhance penetration 5, 1
• Protect surrounding normal skin with petroleum jelly or tape to prevent inadvertent blistering 6

Application Technique

• Apply a thin layer directly to the lesion only using the applicator provided 1, 6
• Allow to air-dry completely (typically 1-2 minutes) before covering or allowing contact with clothing 6
• Do not apply to open wounds, mucous membranes, or areas with broken skin 1

Contact Time (Critical)

• Molluscum contagiosum: 4-6 hours for trunk/extremities; 2-4 hours for face/neck/genitals 6, 4
• Common warts (hands/feet): 4-6 hours 6
• Plantar warts with Canthacur PS: 4-6 hours 2, 4
• Sensitive areas (face, genitals, intertriginous areas): Start with 2-4 hours and adjust based on response 6

Post-Application Instructions

• Patient must wash off thoroughly with soap and water after the specified contact time 1, 6
• Warn patients that blistering will occur 24-48 hours after application—this is expected and therapeutic 1, 2

Treatment Schedule and Reassessment

• Repeat applications every 2-4 weeks until complete clearance 2, 6
• For molluscum contagiosum: Clearance rates range from 15-100%, with most requiring 1-3 treatments 7, 2
• For plantar warts with combination therapy (Canthacur PS): 81-100% clearance, often after a single treatment 2, 4
• Change treatment modality if no substantial improvement after 3-4 applications 8, 6

Expected Adverse Effects

Common and expected reactions include 2, 4:

• Pain at application site (7-85.7% of patients)—generally mild and self-limited 2
• Blistering (10-100%)—this is the therapeutic mechanism and should be anticipated 1, 2
• Hyper- or hypopigmentation (1.8-53.3%)—usually temporary but can persist 2
• Ring warts (Koebner phenomenon)—new warts forming in a ring around the treated site due to viral spread from blister fluid 6

Critical Safety Considerations

Contraindications and Warnings

• Never apply to mucous membranes or use a cryoprobe-style applicator in the vagina 7
• Avoid facial application near eyes—risk of severe ocular injury if cantharidin contacts eyes 6
• Not FDA-approved for any indication (except YCANTH for molluscum as of 2023), though widely used off-label 1, 3
• Highly toxic if ingested—causes GI/GU ulceration, electrolyte disturbances, and renal dysfunction 1
• Store securely away from children and ensure proper patient education about washing off 1, 6

Special Anatomical Considerations

• Genital warts: Use with extreme caution; apply for shorter duration (2-4 hours maximum) 6
• Periungual warts: Protect nail fold and matrix to avoid permanent nail dystrophy 6
• Facial/neck lesions: Use 2-4 hour contact time maximum to minimize scarring risk 6

Comparison to Alternative Treatments

The evidence shows 7, 2:

• Efficacy comparable to cryotherapy for molluscum and warts, but with painless application 2, 6
• Superior patient satisfaction compared to cryotherapy, especially in children 2
• One small RCT (n=29) showed cantharidin improvement over placebo for molluscum was not statistically significant, though open-label studies demonstrate effectiveness 7
• Combination therapy (Canthacur PS) for plantar warts shows higher clearance rates (81-100%) than cantharidin alone 2, 4

Common Pitfalls to Avoid

• Over-application: Applying too much or to too large an area increases adverse effects without improving efficacy 6
• Inadequate patient instruction: Patients must understand to wash off after specified time—failure to do so causes excessive blistering 6
• Treating too frequently: Allow 2-4 weeks between treatments for healing and assessment 6
• Ignoring anatomical location: Sensitive areas require shorter contact times 6
• Not protecting surrounding skin: Leads to unnecessary blistering of normal tissue 6