This very common viral infection can sometimes be difficult to treat due to resistant warts or if a patient has multiple warts. Often surgical options are used in these cases, but this may be undesirable to the patient. In these cases squaric acid used topically is showing to be a good alternative. Squaric acid immunotherapy for warts in children.
https://www.ncbi.nlm.nih.gov/pubmed?term=10775858
Use of squaric acid dibutylester (SADBE) for cutaneous warts in children.
https://www.ncbi.nlm.nih.gov/pubmed?term=10990585