What Are IV Acyclovir Side Effects in Newborn Babies?

High-dose IV acyclovir is a life-saving treatment for neonatal herpes simplex virus (HSV). While it is completely normal to worry about how such strong medication might affect your baby, most infants tolerate treatment well ^[1]. However, serious toxicity is possible, which is why active monitoring is a routine part of NICU care. Because the body relies on the kidneys to process the drug and the bone marrow to produce blood cells, the primary side effects to watch for are kidney stress and lowered blood cell counts.

The Neonatal Intensive Care Unit (NICU) team uses strict, individualized monitoring protocols to detect and manage these issues early, treating the active infection and reducing the risk of complications.

Kidney Stress (Nephrotoxicity)

Acyclovir is cleared from the body through the kidneys, which means it can sometimes cause nephrotoxicity (stress or injury to the kidneys). In a large study of over 1,000 infants on IV acyclovir, acute kidney injury occurred in 5.6% ^[2]. In another study of 89 infants on prolonged treatment, elevated kidney markers occurred in 2% ^[3]. The risk of kidney stress is higher for babies who are critically ill, require a ventilator, or are receiving other medications that can also stress the kidneys ^[2]. Importantly, it was extremely rare for an infant to develop severe kidney failure requiring dialysis in these studies ^[3].

How the NICU monitors and protects the kidneys:

• Careful Fluid Management: One cause of kidney stress is the medication forming tiny crystals. The team helps prevent this by infusing the acyclovir slowly (usually over at least 1 hour) and individualizing your baby’s IV fluids ^[2][4]. Fluid management in newborns is delicate, so the team carefully balances hydration against the risk of fluid overload based on your baby’s heart, lung, and kidney function.
• Lab Tests (Creatinine and BUN): The care team will draw blood to check creatinine and blood urea nitrogen (BUN) ^[2]. These are measurements of waste normally handled by the kidneys. Clinicians look at the trends in these numbers rather than a single result to determine if the kidneys are under stress.
• Urine Output Tracking: Nurses will quantitatively measure your baby’s urine output (often by weighing diapers or using a catheter) and interpret it relative to their body weight and fluid status.
• Dose Adjustments: If kidney stress is detected, doctors can safely adjust the dose of acyclovir or the time between doses to allow the kidneys to recover ^[4].

Low Blood Counts (Neutropenia and Thrombocytopenia)

Acyclovir can sometimes suppress the bone marrow, leading to drops in important blood cells. The two most common concerns are:

• Neutropenia: A drop in the absolute neutrophil count (ANC), which estimates infection-fighting white blood cells.
• Thrombocytopenia: A drop in platelets, the cells that help blood clot. In a study of 89 infants on prolonged high-dose acyclovir, low platelets were observed in 25% of the babies ^[3].

One of the biggest challenges for doctors is that severe HSV infections can also directly cause these same drops in blood cells ^[3][5]. It can be difficult to tell whether the medication or the virus itself is causing the low counts.

How the NICU monitors blood counts:

• Complete Blood Counts (CBC): Your baby will have blood tests to track their ANC and platelet levels.
• Balancing Risks: An abnormal count does not automatically mean acyclovir will be stopped. There is no single universal rule for adjusting treatment, so your baby’s care team will carefully weigh the need to treat the dangerous virus against the risks of low blood counts, considering the severity of the illness and alternative causes ^[1][3].

Other Side Effects and What to Expect

While kidney and blood cell issues are the most closely monitored, parents and nurses also watch for:

• IV Site Inflammation: Swelling or redness where the IV enters the skin.
• Neurologic Symptoms: Rarely, if the kidneys are struggling to clear the drug, it can accumulate and cause marked lethargy or unusual movements.

The exact schedule of blood draws and check-ins varies depending on your baby’s gestational age, weight, illness severity, kidney function, and your hospital’s specific protocols. The plan is highly individualized to ensure your baby gets exactly the monitoring they need.

When to Alert the Nurse:
The team actively manages monitoring, but you should tell your nurse if you notice:

• Markedly less urine or new swelling (rapid fluid-related weight gain).
• Unusual bruising or bleeding.
• Swelling or redness at the IV site.
• New, unusual movements or extreme lethargy.
  (Note: These signs can result from the HSV infection or other illnesses as well as acyclovir, so they should always be interpreted by the clinical team.)