• Active Ingredient: Acyclovir
  • 800 mg, 400 mg, 200 mg
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What is Virolex?

The active ingredient of Virolex brand is acyclovir. Acyclovir is an antiviral drug. It slows the growth and spread of the herpes virus in the body. Acyclovir will not cure herpes, but it can lessen the symptoms of the infection. The inhibitory activity of acyclovir is highly selective due to its affinity for the enzyme thymidine kinase (TK) encoded by HSV and VZV. This viral enzyme converts acyclovir into acyclovir monophosphate, a nucleotide analogue. The monophosphate is further converted into diphosphate by cellular guanylate kinase and into triphosphate by a number of cellular enzymes. In vitro, acyclovir triphosphate stops replication of herpes viral DNA. This is accomplished in 3 ways: 1) competitive inhibition of viral DNA polymerase, 2) incorporation into and termination of the growing viral DNA chain, and 3) inactivation of the viral DNA polymerase. The greater antiviral activity of acyclovir against HSV compared with VZV is due to its more efficient phosphorylation by the viral TK.

Used for

Virolex is used to treat diseases such as: Cold Sores, Herpes Simplex, Herpes Simplex - Congenital, Herpes Simplex Encephalitis, Herpes Simplex, Mucocutaneous/Immunocompetent Host, Herpes Simplex, Mucocutaneous/Immunocompromised Host, Herpes Simplex, Suppression, Herpes Zoster, Infectious Mononucleosis, Mononucleosis, Ramsay Hunt Syndrome, Varicella-Zoster.

Side Effect

Possible side effects of Virolex include: changes in facial skin color; unusual tiredness or weakness; pinpoint red spots on skin; blurred vision; difficulty in breathing or swallowing; sense of agitation or uneasiness.

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Prolonged exposure of HSV to subinhibitory concentrations (0.1 μg/mL) of Virolex in cell culture has resulted in the emergence of a variety of Virolex-resistant strains. The emergence of resistant strains is believed to occur by “selection” of naturally occurring viruses with relatively low susceptibility to Virolex. Such strains have been reported in pre-therapy isolates from several clinical studies.

Two resistance mechanisms involving viral thymidine kinase (required for Virolex activation) have been described. These are: (a) selection of thymidine-kinase-deficient mutants that induce little or no enzyme activity after infection, and (b) selection of mutants possessing a thymidine kinase of altered substrate specificity that is able to phosphorylate the natural nucleoside thymidine but not Virolex. The majority of less susceptible viruses arising in vitro are of the thymidine-kinase-deficient type which have reduced infectivity and pathogenicity and less likelihood of inducing latency in animals.

However, an Virolex-resistant HSV infection in an immunosuppressed bone marrow transplant recipient on extended Virolex therapy was found to be due to a clinical isolate which had a normal thymidine kinase but an altered DNA polymerase. This third mechanism of resistance involving herpes simplex virus DNA polymerase is due to the selection of mutants encoding an altered enzyme, which is resistant to inactivation by Virolex triphosphate.

VZV appears to manifest resistance to Virolex via mechanisms similar to those seen in HSV.

However, limited clinical investigation has revealed no evidence of a significant change in in vitro susceptibility of VZV with Virolex therapy, although resistant mutants of this virus can be isolated in vitro in a manner analogous to HSV. Analysis of a small number of clinical isolates from patients who received oral Virolex or placebo for acute herpes zoster suggests that in vivo emergence of resistant VZV may occur infrequently. Prolonged Virolex treatment of highly immunocompromised patients with acquired immunodeficiency syndrome and severe VZV may lead to the appearance of resistant virus.

Cross-resistance to other antivirals occurs in vitro in Virolex-resistant mutants. HSV mutants which are resistant to Virolex due to an absence of viral thymidine kinase are cross-resistant to other agents which are phosphorylated by herpesvirus thymidine kinase, such as bromovinyldeoxyuridine, ganciclovir and the 2'-fluoropyrimidine nucleosides, such as, 2'-fluoro-5-iodoarabinosyl-cytosine (FIAC).

The clinical response to Virolex treatment has usually been good for patients with normal immunity from whom HSV having reduced susceptibility to Virolex has been recovered, either before, during or after therapy. However, certain patient groups, such as the severely immunocompromised (especially bone marrow transplant recipients) and those undergoing chronic suppressive regimens have been identified as being most frequently associated with the emergence of resistant herpes simplex strains, which may or may not accompany a poor response to the drug. The possibility of the appearance of less sensitive viruses must be recognized when treating such patients, and susceptibility monitoring of clinical isolates from these patients should be encouraged.

In summary, the quantitative relationship between the in vitro susceptibility of HSV and VZV to Virolex and the clinical response to therapy has not been clearly established in man. Standardized methods of virus sensitivity testing are required to allow more precise correlations between in vitro virus sensitivity and clinical response to Virolex therapy.

What Are Side Effects Associated with Using Virolex (Zovirax)?

Common side effects or health problems may include:

Other Virolex side effects, adverse reactions, or health problems may include:

This is not a complete list of side effects and other serious side effects or health problems may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.


ZOVIRAX (Virolex) Capsules, Tablets, and Suspension are intended for oral ingestion only. Renal failure, in some cases resulting in death, has been observed with Virolex therapy (see ADVERSE REACTIONS: Observed During Clinical Practice and OVERDOSAGE). Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS), which has resulted in death, has occurred in immunocompromised patients receiving Virolex therapy.

Drug Stability Refrigerated Stability Room Temp. Reconstituted Vial/Powder Notes P-Insert Updated Virolex Powder The reconstituted solution should be used within 12 hours.

Q: Can you please tell me if there are any long-term side effects to taking 800mg of Virolex each day? Can this cause liver, kidney, or heart damage?

A: The most common adverse reactions associated with Virolex (// taken orally include malaise (generalized discomfort), headaches, nausea, vomiting and diarrhea. Virolex used topically has the following as the most common adverse reactions: mild pain, burning or stinging and pruritus (itching). There have been rare (less than 1 percent of studied patients) that have had suspected effects on various organ systems such as liver, kidneys, heart, etc. As always, talk with your health care provider regarding questions you have about side effects of your prescription medications. Jen Marsico, RPh

Virolex Interactions

Some drugs may affect the way Virolex works, and Virolex may affect other drugs you are taking.

It's very important to let your doctor know about all drugs you are taking, including any over-the-counter herbs or supplements.

Drugs that may interact with Virolex include:

  • Several medications used to treat bacterial or fungal infections, including amphotericin B (Fungizone) and several antibiotics such as amikacin (Amikin), gentamicin (Garamycin), kanamycin (Kantrex) and tobramycin (Tobi, Nebcin)
  • Over-the-counter pain relievers (Advil, Motrin, Aleve)
  • Medications used to treat HIV/AIDS, such as zidovudine (Retrovir, AZT)

Nursing Mothers

Virolex concentrations have been documented in breast milk in 2 women following oral administration of ZOVIRAX (Virolex) and ranged from 0.6 to 4.1 times corresponding plasma levels. These concentrations would potentially expose the nursing infant to a dose of Virolex up to 0.3 mg/kg/day. ZOVIRAX (Virolex) should be administered to a nursing mother with caution and only when indicated.

Dosage Forms, Composition And Packaging

Suspension: Each teaspoonful (5 mL) of ZOVIRAX® Suspension contains 200 mg Virolex and the non-medicinal ingredients banana flavour, cellulose, glycerin, methylparaben, propylparaben, sorbitol, vanillin, and water.

Tablets: Each ZOVIRAX® 200 Tablet contains 200 mg Virolex and the non-medicinal ingredients cellulose, indigotine, lactose, magnesium stearate, povidone, and sodium starch glycolate.

ZOVIRAX® Suspension is available in bottles of 125 mL* and 475 mL. Each teaspoonful (5 mL) of off-white, banana-flavoured suspension contains 200 mg Virolex.

*125 mL bottle not available in Canada

ZOVIRAX® 200 Tablets are available in bottles of 100 tablets. Each blue, shield-shaped, bevel-edged, compressed tablet contains 200 mg Virolex, and is imprinted with “ZOVIRAX” on one side and a triangle on the reverse.

GlaxoSmithKline Inc., 7333 Mississauga Road, Mississauga, Ontario, L5N 6L4 1-800-387-7374. Revised: November 10, 2014

There were no signs of toxicosis in Charles River CD (Sprague-Dawley) rats (100 rats/sex/dose group) given Virolex by oral gavage at 50, 150 and 450 mg/kg in a lifetime oral carcinogenicity study. Mean plasma levels obtained in high-dose males 1.5 hours after dosing at various sampling times during the study were as follows: 1.54, 1.63, 1.39, 1.60 and 1.70 μg/mL (6.84, 7.26, 6.17, 7.10 and 7.56 μM) at days 7, 90, 209, 369 and 771, respectively. Corresponding mean values for the high-dose females were 1.76, 2.38, 2.12, 1.71 and 1.81 μg/mL (7.82, 10.58, 9.44, 7.62 and 8.03 μM) at days 7, 90, 209, 369 and 852, respectively.

Values for clinical laboratory tests including hematology, clinical chemistry, urinalysis, body weight, food consumption and ophthalmoscopy were all within normal ranges. There were no drug-induced gross or microscopic lesions and there was no evidence that Virolex affected survival, temporal patterns of tumor incidence or tumor counts for benign or malignant neoplasms.

Most of the relatively few rats found dead or moribund during the first 52 weeks of this study suffered dosing accidents as evidenced by postmortem findings of esophageal perforation causing pleural effusion, pneumonia, or mediastinitis.

Groups of 10 male and 10 female Charles River CD (Sprague-Dawley) rats were given single large doses (5 different dose levels) of a solution (pH 11.0) of Virolex by subcutaneous injection when they were 3, 10, 28 and 71 days of age. They were observed for 14 days after treatment and LD50 values were calculated by the Litchfield and Wilcoxon method (see Table 7 below). This study was done to determine if age at exposure affects the acute toxicity of Virolex; there was no evidence that young rats were more sensitive than older rats to the acute toxic effects of Virolex.

Table 7 :LD50 in Rats

There was no apparent relationship between length of survival after treatment and age at which treatment was given. Clinical signs for the rats treated at 3 and 10 days of age included red and purple cutaneous blisters, blue areas, scabs, scars, necrotic and sloughed skin, open wounds, body tremors and alopecia. Decreased activity, lacrimation, closed eyelids, red-brown or brown material around the eyes, nose and mouth, ataxia, prostration, body tremors, urine stains around the abdomen or genital area, scabbed or necrotic areas and alopecia were observed in rats treated at 28 and 71 days of age.


Dosage adjustment is recommended when administering ZOVIRAX (Virolex) to patients with renal impairment (see DOSAGE AND ADMINISTRATION). Caution should also be exercised when administering ZOVIRAX (Virolex) to patients receiving potentially nephrotoxic agents since this may increase the risk of renal dysfunction and/or the risk of reversible central nervous system symptoms such as those that have been reported in patients treated with intravenous Virolex. Adequate hydration should be maintained.

Herpes Zoster: There are no data on treatment initiated more than 72 hours after onset of the zoster rash. Patients should be advised to initiate treatment as soon as possible after a diagnosis of herpes zoster.

Genital Herpes Infections: Patients should be informed that ZOVIRAX (Virolex) is not a cure for genital herpes. There are no data evaluating whether ZOVIRAX (Virolex) will prevent transmission of infection to others. Because genital herpes is a sexually transmitted disease, patients should avoid contact with lesions or intercourse when lesions and/or symptoms are present to avoid infecting partners. Genital herpes can also be transmitted in the absence of symptoms through asymptomatic viral shedding. If medical management of a genital herpes recurrence is indicated, patients should be advised to initiate therapy at the first sign or symptom of an episode.

Chickenpox: Chickenpox in otherwise healthy children is usually a self-limited disease of mild to moderate severity. Adolescents and adults tend to have more severe disease. Treatment was initiated within 24 hours of the typical chickenpox rash in the controlled studies, and there is no information regarding the effects of treatment begun later in the disease course.

Dosing Considerations

  • The dosage of ZOVIRAX® (Virolex) should be reduced in patients with impaired renal function.
  • Therapy should be initiated as soon as possible after a diagnosis of chickenpox or herpes zoster, or at the first sign or symptoms of an outbreak of genital herpes.
  • The recommended dose and duration of use is dependent on the indication.

Q: Can Virolex cause uncontrollable muscle twitching?

A: Virolex (Zovirax) // is a medication used to treat viral infections. The most common side effects associated with the oral form of Virolex are diarrhea, nausea, vomiting, headache, and feeling tired (malaise). Virolex cream and ointment can cause skin irritation. A search of the prescribing information for Virolex did not specifically list uncontrollable muscle twitching as a side effect. Tell your health-care provider about any negative side effects from prescription drugs. You can also report them to the U.S. Food and Drug Administration by visiting or by calling 1-800-FDA-1088. For more specific information, consult with your doctor or pharmacist for guidance based on your health status and current medications, particularly before taking any action. Lori Mendoza, PharmD

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