What the label lists and what reports show

Acyclovir side effects

Acyclovir's FDA label lists its adverse reactions in its own words. FAERS holds 114,717 reports naming it through July 30, 2026; the reaction reported most often, leaving out the conditions it is used for, product-use terms and outcomes, is fatigue (8,029 reports).

Generic listed

Check it at the source: the acyclovir FDA label on DailyMed (effective July 24, 2026, Heritage Pharmaceuticals Inc. d/b/a Avet Pharmaceuticals Inc.); the FDA FAERS public dashboard.

What does the patient leaflet say about side effects?

From the patient leaflet: What are the possible side effects of ZOVIRAX Cream?
The most common side effects of ZOVIRAX Cream are skin reactions at the treatment site and may include: dry or cracked lips, peeling, flaking or dryness of the skin, a burning or stinging feeling, and itching. These are not all the possible side effects of ZOVIRAX Cream. Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088.

Source: FDA drug label (Zovirax), Bausch Health US, LLC, effective February 1, 2021 (SPL set 125dbe4e-94ba-45bd-a714-f9c4b8e8b5c2), via openFDA; the full label on DailyMed

What does the label list?

From the label: adverse reactions
Herpes Simplex: Short-Term Administration: The most frequent adverse events reported during clinical trials of treatment of genital herpes with acyclovir 200 mg administered orally 5 timesdaily every 4 hours for 10 days were nausea and/or vomiting in 8 of 298 patient treatments (2.7%). Nausea and/or vomiting occurred in 2 of 287 (0.7%) patients who received placebo. 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 for 1 year in 586 patients treated with acyclovir were nausea (4.8%) and diarrhea (2.4%). The 589 control patients receiving intermittent treatment of recurrences with acyclovir for 1 year reported diarrhea (2.7%), nausea (2.4%), and headache (2.2%). …

Source: FDA drug label, Heritage Pharmaceuticals Inc. d/b/a Avet Pharmaceuticals Inc., effective July 24, 2026 (SPL set 0de2f5c7-9e75-4bdf-9c28-e026cc6694b4), via openFDA; the full label on DailyMed

What do FAERS reports show?

114,717 reports list acyclovir among the medicines taken, each a report of something that happened, not proof the medicine caused it; 14,660 are coded as a death and 42,629 as a hospital stay. Counts follow how widely a medicine is used and watched, and they are not a measure of how risky it is. Of the 22 reactions below, 8 are named by the same term (in US spelling) in the label's adverse reactions, warnings or patient leaflet; a reaction the label does not name may still be there under another word, and a report does not show that the medicine caused it.

Reports received each year

The 25 most reported reactions

Fatigue8,029
Diarrhoea (named in the label)7,346
Off Label Use (a use or a product term, not a reaction)6,289
Pneumonia5,700
Nausea (named in the label)5,645
Pyrexia4,986
Death (an outcome recorded in the report)4,899
Drug Ineffective (a use or a product term, not a reaction)4,744
Rash (named in the label)4,294
Dyspnoea3,636
Asthenia3,361
Headache (named in the label)3,289
Neutropenia3,253
White Blood Cell Count Decreased3,215
Platelet Count Decreased3,118
Constipation2,957
Vomiting (named in the label)2,942
Pain2,905
Dizziness (named in the label)2,857
Febrile Neutropenia2,838
Neuropathy Peripheral2,643
Plasma Cell Myeloma2,611
Anaemia (named in the label)2,575
Acute Kidney Injury2,541
Thrombocytopenia (named in the label)2,459

Source: FDA Adverse Event Reporting System (FAERS) via openFDA, reports through July 30, 2026; reactions as MedDRA preferred terms

What next?

Acyclovir: the full record: label, generics, price, shortages, recalls
Report a side effect to the FDA: MedWatch
Side effects hub: the most reported medicines and reactions