Univir is an antiviral medicine, which acts on infected cells by stopping the virus growing. Univir 400 mg tablets are used to: -treat or prevent herpes simplex infections of the skin and mucous membranes e.g. cold sores and genital herpes (except in children whose immune system works less well and they are unable to fight infections). •prevent herpes simplex infections in patients whose immune system is not working properly. •treat chicken pox (varicella infection).