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

The active ingredient of Activir 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

Activir 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 Activir include: irritability; buccal mucosa tablet; decreased frequency of urination or amount of urine; hives; Burning, prickling, or tingling sensations; headache.

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Activir side effects

Get emergency medical help if you have any signs of an allergic reaction to Activir: hives; difficult breathing; swelling of your face, lips, tongue, or throat.

Call your doctor at once if you have:

easy bruising or bleeding, purple or red pinpoint spots under your skin; or

signs of a kidney problem -little or no urinating; painful or difficult urination; swelling in your feet or ankles; feeling tired or short of breath.

Common Activir side effects may include:

general ill feeling;

mouth pain while using an Activir buccal tablet.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Severe Interactions of Activir include:

  • There are no severe interactions from the use of Activir.

Q: Does Activir cause stomach pain?

A: During clinical trials, patients did not report Activir (Zovirax) causing stomach pain. However, most of the frequently reported adverse reactions associated with Activir treatment were related to the gastrointestinal system. According to the prescribing information for Zovirax, the brand-name equivalent of Activir, 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 Activir, 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 Activir 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 Activir for further evaluation. Activir is an antiviral medication available by prescription only. Activir 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. Activir 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. Activir may be taken without regard to food and should be taken exactly as instructed by a health care provider.

Is Activir safe to take if I'm pregnant or breastfeeding?

There are no adequate studies of Activir in pregnant women. In a patient registry of women who used Activir during the first trimester, the rate of birth defects was similar to the rate of birth defects in the general population.

Activir is excreted in breast milk, and a significant amount may be transferred to the infant.


For management of a suspected drug overdose, contact your regional Poison Control Centre.

Activated charcoal may be administered to aid in the removal of unabsorbed drug. General supportive measures are recommended.

Activir is only partly absorbed in the gastrointestinal tract. Patients have ingested up to 20 g Activir on a single occasion, with no unexpected adverse effects. In clinical studies, the highest plasma concentration observed in a single patient at these doses was 10.0 μg/mL. Accidental, repeated overdoses of oral Activir over several days have been associated with gastrointestinal effects (such as nausea and vomiting) and neurological effects (headache and confusion).

Intravenous doses administered to humans have been as high as 1,200 mg/m² (28 mg/kg) 3 times daily for up to 2 weeks. Peak plasma concentrations have reached 80 μg/mL. Overdosage of intravenous Activir has resulted in elevations of serum creatinine, blood urea nitrogen and subsequent renal failure. Neurological effects including confusion, hallucinations, agitation, seizures and coma have been described in association with intravenous overdosage.

Patients should be observed closely for signs of toxicity. Hemodialysis significantly enhances the removal of Activir from the blood and may, therefore be considered a management option in the event of symptomatic overdose. Precipitation of Activir in renal tubules may occur if the solubility (2.5 mg/mL) in the intratubular fluid is exceeded. In the event of renal failure and anuria, the patient may benefit from hemodialysis until renal function is restored (see DOSAGE AND ADMINISTRATION).

Drug-Drug Interactions

No clinically significant interactions have been identified.

Activir is eliminated primarily unchanged in the urine via active renal tubular secretion. Any drugs administered concurrently that compete with this mechanism may increase Activir plasma concentrations. Probenecid and cimetidine increase the area under the curve (AUC) of Activir by this mechanism, and reduce Activir renal clearance. Similarly, increases in plasma AUCs of Activir and of the inactive metabolite of mycophenolate mofetil, an immunosuppressant agent used in transplant patients have been shown when the drugs are coadministered. However, no dosage adjustment is necessary because of the wide therapeutic index of Activir.

Pediatric Use

Safety and effectiveness of oral formulations of Activir in pediatric patients younger than 2 years of age have not been established.

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