Idoxuridine is a drug that belongs to a class of medicines known as pyrimidine nucleosides. This drug is structurally similar to thymidine and is effective against viral infections. It stops viral replication by substituting itself for thymidine in the DNA of viruses. It is used to treat herpes simples and varicella zoster virus infections. It is effective against vaccinia virus, cytomegalovirus and adenovirus. It is usually applied to the skin and to the eyes as a solution.