Ganciclovir is an anti-viral medication used to treat CMV and severe forms of herpes. It works by inhibiting the formation of DNA and, because of it's mechanism of action, inhibits viral DNA more than normal cellular DNA.
However, because of its mechanism of action, ganciclovir often cause severe side effects, most of which are related to blood cells, but it is also carcinogenic and mutagenic. As such, and because CMV is generally not a life threatening virus, it is only used on patients for whom CMV might be fatal or severely debilitating such as those who are immunocompromised or have AIDS.
Although ganciclovir can be given orally, very little of the drug is absorbed in this way. As such, intravenous administration is more likely. Because CMV can cause blindness in immunocompromised patients, it is also available in a form that can be delivered directly to the eye.