Zovirax (acyclovir) Side Effects, Warnings, and Drug Interactions

Zovirax (acyclovir) is a type of antiviral drug called a synthetic nucleoside analogue, that has inhibitory activity (interferes with viral replication) against the herpes viruses, including herpes simplex 1 and 2 (cold sores and genital herpes), varicella-zoster (shingles and chickenpox), and Epstein-Barr virus (mononucleosis). 

Common side effects of Zovirax include:

• nausea,
• vomiting,
• diarrhea,
• and headache.

Serious side effects of Zovirax include:

• agitation,
• confusion,
• rash,
• anemia,
• muscle pain,
• hypersensitivity reactions,
• seizures,
• and hepatitis.

Drug interactions of Zovirax include:

• phenytoin,
• valproic acid,
• probenecid,
• theophylline, 
• cidofovir,
• and amphotericin B or other drugs that reduce kidney function. 

There are no adequate studies of Zovirax in pregnant women. In a patient registry of women who used Zovirax during the first trimester, the rate of birth defects was similar to the rate of birth defects in the general population. Zovirax is excreted in breast milk, and a significant amount may be transferred to the infant. Consult your doctor before breastfeeding.

The most common side effects are:

• nausea,
• vomiting,
• diarrhea and
• headache.

Other reported side effects include:

• agitation,
• confusion,
• rash,
• anemia, and
• muscle pain,
• hypersensitivity reactions,
• seizures,
• agitation,
• confusion,
• anemia,
• hepatitis, and
• muscle pain.

Adverse Drug Reaction Overview

The most frequent adverse reactions associated with the use of Zovirax® (acyclovir) are headache and nausea.

Neurological side effects have also been reported in rare instances. Elderly patients and patients with a history of renal impairment are at increased risk of developing these effects. In the reported cases, these reactions were generally reversible on discontinuation of treatment.

Clinical Trial Adverse Drug Reactions

Because clinical trials are conducted under very specific conditions the adverse reaction rates observed in the clinical trials may not reflect the rates observed in practice and should not be compared to the rates in the clinical trials of another drug. Adverse drug reaction information from clinical trials is useful for identifying drug-related adverse events and for approximating rates.

Treatment of Herpes Simplex

Short-term administration (5-10 days): The most frequent adverse reactions reported during clinical trials of treatment of genital herpes with oral Zovirax® in 298 patients are listed in Table 1.

Table 1 : Adverse Reactions Reported in Clinical Trials of Treatment of Genital Herpes with Acyclovir

Suppression of Herpes Simplex

Long-term administration: The most frequent adverse events reported in a clinical trial for the prevention of recurrences with continuous administration of 400 mg (two 200 mg capsules) 2 times daily are listed in Table 2.

Table 2 : Adverse Reactions Reported in a Clinical Trial for the Prevention of Recurrences of Genital Herpes with Acyclovir

Evidence so far from clinical trials suggests that the severity and frequency of adverse events is unlikely to necessitate discontinuation of therapy.

Herpes Zoster

The most frequent adverse reactions reported during three clinical trials of treatment of herpes zoster (shingles) with 800 mg of oral Zovirax® 5 times daily for 7 or 10 days or placebo are listed in Table 3.

Table 3 : Adverse Reactions Reported in Clinical Trials of Treatment of Herpes Zoster

Chickenpox

The most frequent adverse events reported during three clinical trials of treatment of chickenpox with oral Zovirax® or placebo are listed in Table 4.

Table 4 : Adverse Reactions Reported in Clinical Trials of Treatment of Chickenpox

Less Common Clinical Trial Adverse Drug Reactions ( < 1%)

Other adverse reactions reported in less than 1% of patients receiving Zovirax® in any clinical trial included: abdominal pain, anorexia, constipation, dizziness, edema, fatigue, flatulence, inguinal adenopathy, insomnia, leg pain, medication taste, skin rash, sore throat, spasmodic hand movement and urticaria.

Abnormal Hematologic And Clinical Chemistry Findings

No clinically significant changes in laboratory values have been observed in clinical trials for the treatment of chickenpox and zoster, and for the treatment and suppression of genital herpes with Zovirax®.

Post-Market Adverse Drug Reactions

The following events have been reported voluntarily during post-market use of Zovirax® in clinical practice. These events have been chosen for inclusion due to either their seriousness, frequency of reporting, potential causal connection to Zovirax® or a combination of these factors. Post-market adverse events are reported spontaneously from a population of unknown size, thus estimates of frequency cannot be made.

General: Fever, headache, pain and peripheral edema.

Nervous: Dizziness, paresthesia, agitation, confusion, tremor, ataxia, dysarthria, hallucinations, psychotic symptoms, convulsions, somnolence, encephalopathy and coma have been reported. These events are generally reversible and usually reported in patients with renal impairment, or with other predisposing factors. These symptoms may be marked, particularly in older adults.

Digestive: Diarrhea, gastrointestinal distress and nausea.

Hematogical and Lymphatic: Anaemia, leukopenia, lymphadenopathy and thrombocytopenia.

Hypersensitivity and Skin: Alopecia, erythema multiforme, Stevens-Johnson syndrome, toxic epidermal necrolysis, rashes including photosensitivity, pruritus, urticaria, dyspnoea, angioedema and anaphylaxis.

Hepatobiliary Tract and Pancreas: Reports of reversible hyperbilirubinemia and elevated liver related enzymes. Hepatitis and jaundice.

Musculoskeletal: Myalgia.

Special Senses: Visual abnormalities.

Urogenital: Elevated blood creatinine and blood urea nitrogen (BUN). Acute renal failure, renal pain and hematuria have been reported. Renal pain may be associated with renal failure.

Drug-Drug Interactions

No clinically significant interactions have been identified.

Acyclovir is eliminated primarily unchanged in the urine via active renal tubular secretion. Any drugs administered concurrently that compete with this mechanism may increase acyclovir plasma concentrations. Probenecid and cimetidine increase the area under the curve (AUC) of acyclovir by this mechanism, and reduce acyclovir renal clearance.

Similarly, increases in plasma AUCs of acyclovir 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 acyclovir.

Drug-Food Interactions

There is no known interaction with food.

Drug-Herb Interactions

Interactions with herbal products have not been established.

Drug-Laboratory Test Interactions

Interactions with laboratory tests have not been established.