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

The active ingredient of Clirbest 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

Clirbest 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 Clirbest include: pinpoint red spots on skin; decreased consciousness; coughing; hives; changes in vision; increased thirst; difficulty in breathing or swallowing.

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• Make sure patient is adequately hydrated before starting therapy.

• Give single I.V. dose by infusion over at least 1 hour to minimize renal damage.

• Don't give by I.V. bolus or by I.M. or subcutaneous route.

• Be aware that absorption of topical Clirbest is minimal.

Q: Is there an Clirbest ointment?

A: There is an Clirbest (Zovirax) ointment formulation, available in 5% strength. Currently, Clirbest ointment is only available as the brand-name Zovirax Ointment 5%. Clirbest ointment is indicated for the management of initial genital herpes and in limited non-life threatening mucocutaneous herpes simplex virus infections in immunocompromised patients. Clirbest ointment is not indicated for the prevention of viral transmission to other individuals. Clirbest ointment is also not indicated for the prevention of recurrent herpes infections. Clirbest ointment is a prescription ointment that is applied directly to genital herpes lesions and may lessen the duration of the symptoms associated with an initial outbreak. For the management of genital herpes, patients should be instructed regarding appropriate dosage and administration of Clirbest ointment. Patients should be advised to apply a sufficient quantity of Clirbest ointment to adequately cover all lesions and apply every 3 hours, 6 times daily, for 7 days of treatment. The dose size of Clirbest ointment for each application may vary depending on the size of the lesion area. However, the dose size should be approximately ½ inch ribbon of Clirbest ointment per 4 square inches of surface area, according to the prescribing information. Patients should be advised to wear a rubber glove or finger cot when applying Clirbest ointment to the affected area to avoid spreading the virus. Clirbest ointment should be applied as early as possible after the development of signs and symptoms. During controlled clinical trials, mild pain, including temporary burning and stinging, was reported in approximately 30% of patients with no significant difference between patients receiving Zovirax ointment and patients receiving placebo. Local pruritus was also reported in 4% of patients. Clirbest ointment 5% should be used exactly as prescribed by a health care provider. Patients should be advised not to exceed the recommended dosage, frequency of applications and length of treatment.

Clirbest - 12 years past I had gential warts. I have never had a breakout since they where chemicly

Updated 15 Feb 2013 • 2 answers

Sun sensitivity

Clirbest can make your skin more sensitive to the sun. This increases your risk of sunburn. Avoid the sun if you can. If you can’t, be sure to wear protective clothing and apply sunscreen.

Caution is advised when administering Clirbest to patients with impaired renal function. Adequate hydration should be maintained.

Comprehensive pharmacokinetic studies have been completed following intravenous Clirbest infusions in patients with renal impairment.

Based on these studies, dosage adjustments are recommended in Table 5 for genital herpes and herpes zoster indications.

Table 5 : Dosage Modification for Renal Impairment

Serious Interactions of Clirbest include:

  • amphotericin B deoxycholate
  • bacitracin
  • cidofovir
  • neomycin PO
  • oral probenecid
  • talimogene laherparepvec

Clirbest has moderate interactions with at least 28 different drugs.Clirbest has mild interactions with at least 65 different drugs.This document does not contain all possible interactions from the use of this medication. Therefore, before using this drug, tell your doctor or pharmacist of all the drugs you use. Keep a list of all your medications with you, and share the list with your doctor and pharmacist. Check with your physician if you have health questions or concerns.

What Other Drugs Interact with Clirbest (Zovirax)?

If your doctor has directed you to use this medication, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of this medicine or any drug before seeking medical advice from your doctor, healthcare provider or pharmacist first. To do so may result in serious consequences or side effects.

Q: Does Clirbest cause stomach pain?

A: During clinical trials, patients did not report Clirbest (Zovirax) causing stomach pain. However, most of the frequently reported adverse reactions associated with Clirbest treatment were related to the gastrointestinal system. According to the prescribing information for Zovirax, the brand-name equivalent of Clirbest, the most frequently reported gastrointestinal adverse reactions reported by patients during clinical trials for any indication included nausea, vomiting and diarrhea. In addition, post marketing experience revealed other gastrointestinal related adverse reactions observed in clinical practice. In clinical practice experience with Clirbest, patients reported experiencing diarrhea, gastrointestinal distress and nausea. There was no further description regarding gastrointestinal distress and whether or not patients experienced stomach pain specifically. Other common adverse reactions possible with Clirbest treatment include headache and malaise. It is essential for patients to consult a health care provider if they experience any unusual or bothersome adverse reactions while being treated with Clirbest for further evaluation. Clirbest is an antiviral medication available by prescription only. Clirbest is indicated for the acute treatment of shingles, the treatment of initial episodes and the management of recurrent episodes of genital herpes and the treatment of chickenpox. Clirbest is approved for use in adults for all indications and for the treatment of chickenpox in the pediatric population in children 2 years of age and older. Clirbest may be taken without regard to food and should be taken exactly as instructed by a health care provider.

Adult Mice and Rats: The acute toxicity of oral Clirbest was determined as shown in Table 6.

Table 6 : Acute Toxicity Studies


ZOVIRAX® (Clirbest) is contraindicated for patients who develop hypersensitivity or who are hypersensitive to Clirbest, valClirbest or any other components of the formulations of ZOVIRAX®. For a complete listing, see Dosage Forms, Composition and Packaging section of the product monograph.

Dosing Considerations

  • The dosage of ZOVIRAX® (Clirbest) 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.

Why is this medication prescribed?

Clirbest is used to decrease pain and speed the healing of sores or blisters in people who have varicella (chickenpox), herpes zoster (shingles; a rash that can occur in people who have had chickenpox in the past), and first-time or repeat outbreaks of genital herpes (a herpes virus infection that causes sores to form around the genitals and rectum from time to time). Clirbest is also sometimes used to prevent outbreaks of genital herpes in people who are infected with the virus. Clirbest is in a class of antiviral medications called synthetic nucleoside analogues. It works by stopping the spread of the herpes virus in the body. Clirbest will not cure genital herpes and may not stop the spread of genital herpes to other people.

Missed Dose of Clirbest

You should take Clirbest exactly as directed by your doctor. Also, do not stop taking Clirbest on your own.

Skipping doses or stopping too soon may not completely treat your infection, or it could make the infection harder to treat.

If you miss a dose of Clirbest, take the missed dose as soon as you remember.

If it's almost time for your next dose, skip the missed dose.

Do not double your dose to make up for a missed dose.

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

Two resistance mechanisms involving viral thymidine kinase (required for Clirbest 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 Clirbest. 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 Clirbest-resistant HSV infection in an immunosuppressed bone marrow transplant recipient on extended Clirbest 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 Clirbest triphosphate.

VZV appears to manifest resistance to Clirbest 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 Clirbest 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 Clirbest or placebo for acute herpes zoster suggests that in vivo emergence of resistant VZV may occur infrequently. Prolonged Clirbest 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 Clirbest-resistant mutants. HSV mutants which are resistant to Clirbest 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 Clirbest treatment has usually been good for patients with normal immunity from whom HSV having reduced susceptibility to Clirbest 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 Clirbest 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 Clirbest therapy.

Why it's used

Clirbest is used to treat viral infections. These include varicella-zoster (shingles), genital herpes, and chickenpox.

This drug doesn't cure herpes infections. The herpes virus can stay in your body for a long time and cause symptoms again later.

Q: Can Clirbest cause uncontrollable muscle twitching?

A: Clirbest (Zovirax) // is a medication used to treat viral infections. The most common side effects associated with the oral form of Clirbest are diarrhea, nausea, vomiting, headache, and feeling tired (malaise). Clirbest cream and ointment can cause skin irritation. A search of the prescribing information for Clirbest 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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