Zovirax Side Effects

Generic Name: acyclovir

Note: This page contains side effects data for the generic drug acyclovir. It is possible that some of the dosage forms included below may not apply to the brand name Zovirax.

It is possible that some side effects of Zovirax may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

For the Consumer

Applies to acyclovir: capsule, powder for solution, solution, suspension, tablet

Other dosage forms:

As well as its needed effects, acyclovir (the active ingredient contained in Zovirax) may cause unwanted side effects that require medical attention.

If any of the following side effects occur while taking acyclovir, check with your doctor immediately:

More common - For acyclovir injection only
  • Pain, swelling, or redness at place of injection
Less common
  • Abdominal or stomach pain
  • decreased frequency of urination or amount of urine
  • increased thirst
  • loss of appetite
  • nausea or vomiting
  • unusual tiredness or weakness
  • Black, tarry stools
  • blood in urine or stools
  • chills, fever, or sore throat
  • confusion
  • convulsions (seizures)
  • hallucinations (seeing, hearing, or feeling things that are not there)
  • hives
  • pinpoint red spots on skin
  • trembling
  • unusual bleeding or bruising
Frequency not determined
  • Bleeding or oozing from puncture sites or mucous membranes (bowel, mouth, nose, or urinary bladder), continuing
  • blistering, peeling, or loosening of skin
  • bluish coloring, especially of the hands and feet
  • blurred vision
  • bruising at the place of injection
  • changes in facial skin color
  • changes in vision
  • clumsiness
  • coughing
  • decreased consciousness
  • difficulty in breathing or swallowing
  • dizziness or feeling faint, severe
  • fast heartbeat
  • irritability
  • itching or skin rash
  • large hive-like swelling on face, eyelids, lips, tongue, throat, hands, legs, feet, sex organs
  • mood or mental changes
  • muscle cramps, pain, or weakness
  • pale skin
  • red or irritated eyes
  • sense of agitation or uneasiness
  • shakiness and unsteady walk
  • sores, ulcers, or white spots in mouth or on lips
  • swelling of eyelids, face, feet, hands, lower legs or lips
  • swollen, painful, or tender lymph nodes (glands) in neck, armpit, or groin
  • unsteadiness or other problems with muscle control or coordination
  • yellow eyes or skin

Some acyclovir side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned about them:

More common - Especially seen with high doses
  • General feeling of discomfort or illness
Less common - Especially seen with long-term use or high doses
  • Diarrhea
  • headache
Frequency not determined
  • Burning, prickling, or tingling sensations
  • drowsiness
  • loss of hair

For Healthcare Professionals

Applies to acyclovir: buccal tablet, compounding powder, intravenous powder for injection, intravenous solution, oral capsule, oral suspension, oral tablet


Gastrointestinal side effects have been the most frequently reported side effects, and include nausea, vomiting, abdominal pain, and diarrhea.

Nausea and vomiting have been reported with oral and intravenous administration, and have preceded neurotoxicity and nephrotoxicity. Gagging and anorexia have also been reported.


Renal side effects have included renal failure, renal pain (may be associated with renal failure), elevated blood urea nitrogen, elevated serum creatinine, and hematuria. Renal effects generally are transient and resolve over several days following discontinuation of therapy; however fatal renal failure has occurred. Renal damage is most commonly due to crystallization of the drug in the renal tubules. Acute tubular necrosis and interstitial nephritis have also been reported.

Transient renal dysfunction has been reported with both oral and intravenous acyclovir therapy. Crystallization of the drug in renal tubules is thought to be the mechanism for the development of renal dysfunction, based on findings of crystalluria in several case reports and at least one prospective study.

Elderly or renally impaired patients are at greater risk for developing neurotoxicity and further deterioration in renal function.

Nervous system

Nervous system side effects have included aggressive behavior, agitation, ataxia, coma, confusion, decreased consciousness, delirium, delusions, disorientation, dizziness, EEG changes, abnormal cerebrospinal fluid findings, encephalopathy, focal neurological signs, hallucinations, headache, insomnia, irritability, lightheadedness, major depression, mania, myoclonus, obtundation, paresthesia, psychosis, seizure, somnolence, tremors, and Cotard's syndrome. Neurotoxicity generally develops early in acyclovir (the active ingredient contained in Zovirax) treatment and has most commonly been reported in patients with renal failure, the elderly, and in patients following bone marrow transplant. It is thought to be associated with high serum concentrations of acyclovir. Guillain Barre syndrome has been reported in at least one patient receiving acyclovir prophylaxis following allogeneic marrow transplantation.

Acyclovir neurotoxicity is almost exclusively seen in patients with renal failure. These patients may have longstanding chronic renal failure or acute failure associated with acyclovir. Although more commonly seen with intravenous administration of larger doses, neurotoxicity has also been reported in patients receiving oral doses of acyclovir. Following discontinuation of acyclovir, mental status recovers within about a week. Several patients with chronic renal failure exhibiting neurotoxicity improved dramatically following hemodialysis. In one study of renal transplant patients receiving oral acyclovir therapy, one patient developed neurotoxicity, manifested as disorientation, confusion and myoclonus. The symptoms responded to a decrease in dosage. Three other case reports have also described neurologic symptoms, including visual hallucinations, delusions, mania, tremors, myoclonus and EEG changes, which improved following discontinuation of intravenous acyclovir. Rechallenge in one case using a lower dosage resulted in no sequelae.


Local adverse effects associated with intravenous administration of acyclovir (the active ingredient contained in Zovirax) have included inflammation or phlebitis at the injection site. Phlebitis is more common when concentrated solutions (greater than 7 mg/mL) are administered. Skin eruptions have been reported at venipuncture sites and tissue necrosis has occurred after infiltration into extravascular tissues.


Cardiovascular side effects have included hypotension.


Dermatologic side effects have included alopecia, erythema multiforme, hives, photosensitive rash, pruritus, rash, Stevens-Johnson syndrome, toxic epidermal necrolysis, and urticaria.


Hematologic and lymphatic side effects have included anemia, disseminated intravascular coagulation, hemolysis, leukocytoclastic vasculitis, leukocytosis, leukopenia, lymphadenopathy, neutropenia, neutrophilia, thrombocytosis, thrombocytopenia, and pancytopenia.


Hepatic side effects have included elevated liver function tests, hepatitis, hyperbilirubinemia, and jaundice.


Hypersensitivity reactions have included anaphylaxis.


Ocular side effects have included visual abnormalities.


Musculoskeletal side effects reported have included myalgia and dysarthria.


Other side effects have included angioedema, fever, malaise, pain, fatigue, peripheral edema, and increased lactate dehydrogenase.


Genitourinary side effects have included crystalluria.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. Drugs.com does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.