Formaldehyde

How it works

It is a chemical caustic and protein cross-linking agent that denatures keratin and macerates or destroys infected hyperkeratotic tissue.

Prescribing in practice

• It is corrosive and must be confined to the lesion; protect surrounding healthy skin and avoid mucous membranes, the face and broken skin to prevent chemical burns.
• It is a recognised contact sensitiser, so discontinue if allergic contact dermatitis develops.
• It is not suitable for use on mosaic warts of the soles in the same way as solitary lesions and should be used with particular care in diabetes or peripheral arterial disease.

Monitoring

No laboratory monitoring is required; review the lesion response and inspect surrounding skin for irritation or sensitisation.

Counselling the patient

• Apply only to the wart and protect the surrounding skin, for example with soft paraffin.
• Stop and seek advice if the skin becomes painful, blistered or develops a rash.
• Keep away from the eyes, mouth and healthy skin.

Evidence & guidelines

Topical keratolytic and caustic agents are long-standing options for cutaneous warts; most warts also resolve spontaneously over time.