Inclusion Criteria:

• Outpatient males or females 12 to <18 years of age
• General good health with a documented history typical for recurrent herpes labialis

• Prodromal symptoms or active lesions suggestive of a recurrent episode of herpes labialis (i.e. having had cold sores in the past) , with onset not exceeding 24 hours until the time of study drug administration

  • Adolescents participating in PK part of the study may be enrolled without an active herpes labialis recurrence or with onset of signs/symptoms of a recurrent herpes labialis episode longer than 24 hours before study drug administration, All adolescents participating in the pharmacokinetics assessments must fast for at least 8 hours prior to Visit 1 and be willing to fast for an additional 2 hours after study drug administration

Exclusion Criteria:

• Use of other investigational drugs within 30 days of enrollment
• History of hypersensitivity to famciclovir or penciclovir
• Inability to swallow tablets
• Body weight less than 40 kg
• History of malabsorption, unless a condition like celiac disease is stable and well controlled, previous gastrointestinal surgery or radiation therapy that could affect drug absorption or metabolism, or any condition that could interfere with drug absorption, distribution, metabolism, or excretion
• Known renal insufficiency (calculated creatinine clearance <60 mL/min)
• Known severe hepatic impairment (Child-Pugh Class C)
• Significant skin disease such as atopic dermatitis or eczema that would interfere with assessment of oral/labial lesions
• Known to be immunocompromised or are receiving systemic or using topical immunosuppressive agents (including corticosteroids, tacrolimus and picrolimus) within 30 days of enrollment
• Concomitant use of probenecid
• Pregnant or nursing (lactating) females
• Females of child-bearing potential, UNLESS they are using two birth control methods. The two methods can be a double barrier method or a barrier method plus a hormonal method.