What are the risks of using acyclovir in immunocompromised patients?

What risks show up most often in practice?
Acyclovir can cause kidney damage in immunocompromised patients when given at high intravenous doses or when patients are dehydrated. The drug crystallizes in renal tubules, producing acute tubular necrosis or interstitial nephritis that sometimes requires dialysis.

How common is neurotoxicity and what triggers it?
Confusion, tremors, hallucinations, and seizures occur in roughly 1 % of severely immunocompromised patients, especially those with impaired renal clearance or low body weight. Symptoms usually reverse within days after the drug is stopped or the dose is reduced.

Can resistance develop and what does it mean for treatment?
Prolonged or repeated courses select for thymidine-kinase-deficient herpes strains. Resistant isolates appear in 5–10 % of bone-marrow or solid-organ transplant recipients and force a switch to foscarnet or cidofovir, both of which carry greater toxicity.

Does the drug interact with other transplant medicines?
Concurrent use with cyclosporine, tacrolimus, or other nephrotoxic agents amplifies the risk of renal failure. Monitoring creatinine every 48–72 hours and adjusting the acyclovir dose to estimated glomerular filtration rate are standard safeguards.

When is dose adjustment required?
Any drop in creatinine clearance below 50 mL/min calls for a 50 % dose reduction; below 10 mL/min the interval is lengthened to every 24 hours. Oral formulations need similar downward titration in patients with moderate-to-severe renal impairment.

Are there special concerns in children or the elderly?
Pediatric oncology patients on chemotherapy show higher rates of both nephrotoxicity and neurotoxicity because of immature or fluctuating renal function. Elderly transplant recipients often have baseline chronic kidney disease, so the same dose-reduction rules apply early.

How is monitoring typically done?
Daily serum creatinine, urine output, and neurologic checks are routine during intravenous therapy. Some centers also measure acyclovir trough levels when renal function is unstable or when high-dose regimens are used for disseminated disease.