Idoxuridine is the oldest antiviral drug used as a topical ophthalmic preparation. It is a derivative of thymidine, one of the organic bases that are incorporated into the nucleic acids.

Mode of action:

Idoxuridine irreversively inhibits incorporation of thymidine into viral DNA and replaces thymidine. So the newly formed viral particles are ineffective.


Idoxuridine exhibits poor penetration into the eye. Epithelial lesions are most responsive to therapy, while infections with stromal involvement are less responsive.  Idoxuridine is used as 0.1% eye drops and 0.5% eye ointment.

Anti-viral profile:

It is indicated for use in treatment of herpes simplex keratitis. Idoxuridine is ineffective against herpes simplex virus type 2 and varicella-zoster.

Adverse effect:

The drug is only used topically because of its cardiotoxicity on systemic use. When applied topically it is relatively safe, though corneal toxicity can arise if used for prolonged periods. For this reason, it is not used for more than 21 days. It inhibits stromal healing and causes a reduction of wound strength and so cannot be used after corneal transplants.