抗病毒藥品
1. Initial and recurrent episodes of genital herpes
2. Acute treatment of herpes zoster (shingles)
3. Treatment of varicella (chickenpox)

Antiviral Agent

Absorption:
1. Poorly absorbed; absorption improves with multiple small doses compared to one large daily dose
2. Bioavailability: 10% to 20% with normal renal function (bioavailability decreases with increased dose)
Distribution:
1. Widely (eg, brain, kidney, lungs, liver, spleen, muscle, uterus, vagina, CSF); CSF acyclovir concentration is ~50% of plasma concentrations
2. Protein binding: 9% to 33%
Metabolism:
Converted by viral enzymes to acyclovir monophosphate, and further converted to diphosphate then triphosphate (active form) by cellular enzymes
Excretion:
Urine (62% to 91% as unchanged drug and metabolite)

Hypersensitivity to acyclovir, valacyclovir, or any component of the formulation

Acyclovir has been shown to cross the human placenta

Acyclovir is present in breast milk.

Common:
Decreased hemoglobin, decrease in absolute neutrophil count, Malaise
Others:
1. Dermatologic: Pruritus, skin rash, urticaria
2. Gastrointestinal: Diarrhea, nausea, vomitin
3. Hematologic & oncologic: Thrombocytopenia
4. Hepatic: Increased serum bilirubin and transaminases
5. Local: Inflammation at injection site, injection site phlebitis
6. Nervous system: Headache
7. Renal: Increased blood urea nitrogen, increased serum creatinine

Herpes zoster (shingles):
800 mg 5 times daily for 7 days, Initiate at earliest sign or symptom; treatment is most effective when initiated ≤72 hours after rash onset, but may initiate treatment >72 hours after rash onset if new lesions are continuing to appear
Varicella (chickenpox):
800 mg 5 times daily for >5 to 7 days and until all lesions have crusted

Herpes zoster (shingles):
800 mg every 4 hours (5 doses per day) for 5 to 7 days
Varicella (chickenpox):
20 mg/kg/dose 4 times daily for 5 days; maximum 3,200 mg/day

1. If the usual dose is 400 mg every 12 hours or 200 mg 5 times daily:
(1) CrCl >25: No dosage adjustment necessary
(2) CrCl 10-25: No dosage adjustment necessary or reduce to 200 mg Q12H
(3) CrCl<10: 200 mg Q12H

2. If the usual dose is 800 mg 5 times daily:
(1) CrCl >25: No dosage adjustment necessary.
(2) CrCl 10-25: 800 mg Q8H
(3) CrCl<10: 200 mg every 12 hours or 400 mg every 12 hours

There are no dosage adjustments provided in the manufacturer's labeling; use caution in patients with severe impairment.