Famciclovir is another nucleoside analog that is a prodrug of penciclovir. Approximately 80% of the drug is absorbed with oral dosing. Its longer half-life also permits less frequent dosing than with acyclovir. Headache, nausea, and diarrhea occurred in more than 3% (Turkoski et al,1999). In animal studies with rats, mice, and dogs, testicular toxicity was observed related to the dose and duration of exposure to famiciclovir (Turkoski et al). These findings have not been replicated in humans (Abramowicz & Rizack, 1997). As with acyclovir and valacyclovir, the response to alterations in TK renders famciclovir ineffective in the treatment of resistant HSV