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

Zovirax (acyclovir) is generally a well-tolerated antiviral medication, and most people experience few or no significant side effects. However, like all medications, it carries risks — some minor and some serious. Understanding what to watch for will help you use the medication safely and know when to seek medical attention.

Common Side Effects of Oral Zovirax (Acyclovir)

The most frequent side effects of oral acyclovir tablets, capsules, and suspension include:

Headache — the most commonly reported side effect in clinical trials

Nausea — reported in about 4.8% of patients on long-term suppressive therapy

Vomiting

Diarrhea — reported in about 2.4% of long-term users

Malaise (general feeling of being unwell)

Taking acyclovir with food and a full glass of water can help reduce nausea. These side effects are usually mild and tend to resolve on their own. They are rarely severe enough to require stopping the medication.

Side Effects of Topical Zovirax (Cream and Ointment)

Zovirax cream and ointment are applied to the skin and are minimally absorbed into the body. Local side effects at the application site can include:

Mild burning or stinging

Itching

Dryness or flaking of treated skin

Mild redness or irritation

Systemic side effects from topical acyclovir are rare because absorption through the skin is minimal. Do not apply topical forms to eyes, inside the mouth, nose, or genitals unless specifically prescribed for those areas.

Serious Side Effects: When to Seek Immediate Help

While rare, acyclovir can cause serious side effects that require immediate medical attention:

Kidney damage (acute renal failure): Acyclovir can crystallize in the kidneys, especially if you are dehydrated. Signs include decreased urination, swelling in your legs or feet, or unusual fatigue. Drink plenty of fluids (6–8 glasses of water daily) while taking acyclovir.

CNS effects (rare): Confusion, hallucinations, drowsiness, tremors, or seizures. These are more likely in elderly patients or those with kidney impairment. Report these symptoms immediately.

Severe skin reactions: Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), DRESS syndrome, and AGEP have been reported with acyclovir. Seek emergency care immediately if you develop a painful rash involving your mouth, eyes, or genitals, or a widespread blistering rash. Stop the medication immediately.

TTP/HUS (thrombotic thrombocytopenic purpura/hemolytic uremic syndrome): A rare but life-threatening blood disorder reported in immunocompromised patients receiving acyclovir. Symptoms include unusual bruising, blood in urine, and severe fatigue.

Severe allergic reactions: Stop taking acyclovir and seek emergency care immediately if you experience hives, difficulty breathing, swelling of your face, lips, tongue, or throat.

Who Is at Higher Risk for Side Effects?

Older adults: More likely to have reduced kidney function, which means acyclovir clears more slowly from the body. Nausea, vomiting, dizziness, and CNS effects are more common in elderly patients. Dose reduction may be needed.

Kidney disease: Dose adjustment is required for patients with reduced renal function. Your doctor will modify the dose and frequency based on your creatinine clearance.

Immunocompromised patients: Higher risk for TTP/HUS and serious CNS events. Extra monitoring is recommended.

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Safety Tips for Taking Zovirax

Drink 6–8 glasses of water per day while taking oral acyclovir to protect your kidneys

You may take acyclovir with or without food — food does not affect absorption

Take each dose with a full 8-ounce glass of water

Never double-dose if you miss a dose — skip it if it's close to the next scheduled dose

Tell your doctor about all other medications you're taking, especially other drugs that affect kidney function

When to Call Your Doctor

Call your doctor if nausea or diarrhea is severe or doesn't improve, if you have trouble urinating or notice swelling, if you feel confused or unusually drowsy, or if your symptoms don't start improving within a few days of starting treatment. For a full list of drug interactions to watch for, see our guide on Zovirax drug interactions.