Adults

Herpes Zoster (Shingles): 800 mg five times daily for 7 to 10 days. Varicella (Chickenpox): 800 mg four times daily for 5 days. Treatment of HSV: 200 mg five times daily for 5 days. Suppression of recurrent HSV: 400 mg twice daily.

Elderly

Careful dosage adjustment is advised in elderly patients, especially those with impaired renal function. Hydration should be maintained.

Renal_impairment

Dosage adjustment is necessary based on creatinine clearance. For CrCl 10-25 mL/min: 800 mg every 8 hours for Zoster; 200 mg every 8 hours for HSV. For CrCl <10 mL/min: 800 mg every 12 hours for Zoster; 200 mg every 12 hours for HSV.

Onset

Variable, generally within 24-48 hours for symptomatic relief, but full antiviral effect may take longer.

Excretion

Mainly excreted unchanged in the urine by glomerular filtration and tubular secretion. Renal clearance is directly related to creatinine clearance.

Half life

Approximately 2.5 to 3.3 hours in adults with normal renal function. It is longer in patients with renal impairment.

Absorption

Poorly absorbed from the gastrointestinal tract; bioavailability is approximately 10-20%. Peak plasma concentrations are reached within 1.5 to 2 hours after oral administration.

Metabolism

Primarily metabolized to an inactive metabolite, 9-carboxymethoxymethylguanine, which accounts for approximately 10-15% of the dose.

Probenecid

Increases aciclovir plasma half-life and AUC by reducing renal tubular secretion. Concurrent use may require dosage adjustment.

Nephrotoxic drugs

Concurrent use with other nephrotoxic drugs may increase the risk of renal dysfunction. Close monitoring of renal function is recommended.

Mycophenolate Mofetil

Increases plasma AUC of both aciclovir and the inactive metabolite of mycophenolate mofetil. Monitor for signs of toxicity.