Valganciclovir Side Effects: What to Expect and When to Call Your Doctor

Valganciclovir (Valcyte) is a powerful antiviral medication that carries significant risks alongside its benefits. The FDA has issued multiple boxed warnings — the agency's most serious category of alert — for this drug. Understanding what side effects to watch for, and which ones require immediate medical attention, is essential for everyone taking valganciclovir.

FDA Boxed Warnings: The Most Serious Risks

The FDA requires the following boxed warnings on valganciclovir labeling — these represent risks serious enough to cause severe injury or death:

• Hematologic (blood) toxicity: Severe leukopenia (low white blood cells), neutropenia (low infection-fighting cells), anemia (low red blood cells), thrombocytopenia (low platelets), pancytopenia, bone marrow failure, and aplastic anemia. These can develop at any time during treatment.
• Teratogenicity (birth defects): Based on animal data, valganciclovir has the potential to cause birth defects in humans. Women must use effective contraception during treatment and for 30 days after the last dose. Men must use condoms during treatment and for 90 days after the last dose.
• Carcinogenicity (cancer risk): Animal data show ganciclovir (the active form of valganciclovir) is carcinogenic. The drug should be considered a potential human carcinogen.
• Impairment of fertility: May cause temporary or permanent inhibition of sperm production in males. May suppress fertility in females. Patients planning future pregnancies should discuss this with their physician before starting therapy.

Common Side Effects (≥20% of Patients)

The most frequently reported side effects in clinical trials — occurring in at least 20% of adult patients — include:

• Diarrhea
• Fever (pyrexia)
• Nausea
• Tremor
• Neutropenia (low white blood cells — a lab value, often asymptomatic until severe)
• Anemia
• Leukopenia
• Thrombocytopenia (low platelets)
• Headache
• Insomnia
• Vomiting and abdominal pain

Taking valganciclovir with food can help reduce nausea, vomiting, and gastrointestinal side effects. This is also recommended by the prescribing label to improve drug absorption.

When to Call Your Doctor Right Away

Contact your prescriber promptly — same day — if you experience any of the following:

• Signs of infection: fever over 100.4°F (38°C), chills, flu-like symptoms, sore throat, mouth sores, or unusual tiredness — these can signal dangerously low white blood cell counts
• Unusual bruising or bleeding that won't stop — sign of low platelets (thrombocytopenia)
• Extreme fatigue, weakness, shortness of breath, or pale skin — signs of anemia
• Little or no urination, swelling in feet or ankles — signs of kidney damage
• Changes in vision — may indicate CMV retinitis is not responding to treatment or worsening

When to Go to the Emergency Room

Go to the ER immediately or call 911 if you experience:

• Signs of severe allergic reaction: hives, difficulty breathing, swelling of the face/throat
• Severe chest pain or difficulty breathing
• Seizures
• Sudden significant blood in urine or stool

Why Blood Count Monitoring Is Critical

Because blood toxicity is the most common serious adverse effect of valganciclovir, your doctor will order regular complete blood counts (CBCs) while you are on this medication. Do not skip these lab appointments. Neutropenia (low neutrophils) and thrombocytopenia (low platelets) often have no obvious symptoms until they become severe. Catching them early on a blood test allows your doctor to adjust or temporarily stop your dose before a dangerous situation develops. Blood counts typically begin to recover within 3–7 days of stopping the medication if a dose hold is needed.

To learn about which medications can make valganciclovir more dangerous to take, see: Valganciclovir Drug Interactions: What to Avoid.

If you are having trouble filling your valganciclovir prescription, medfinder can help locate a pharmacy near you that has it in stock.