Duofilm contains two medicines called salicylic acid & lactic acid. It works by softening the skin of a wart, causing it to peel off. Duofilm is used to treat warts. A wart is an infection in the top layers of the skin. It is caused by a virus. Warts can be spread when the virus touches damaged skin. It is more likely to spread in places which are warm & moist. This is why one of the most common places to catch warts is swimming pools. It can take from a few weeks to several months for the wart to appear