Acyclovir side effects: what 1,249 FDA adverse event reports show
Drug class
Herpes Simplex Virus Nucleoside Analog DNA Polymerase Inhibitor, Herpes Zoster Virus Nucleoside Analog DNA Polymerase Inhibitor, Herpesvirus Nucleoside Analog DNA Polymerase Inhibitor
Who reports
1,249 reports · 83% serious · 189 with death outcome · 57% female
Used for
Herpes Zoster Infections: Acyclovir capsules are indicated for the acute treatment of herpes zoster (shingles). Genital Herpes: Acyclovir capsules are indicated for the treatment of initial episodes and the management of recurrent episodes of genital herpes. Chickenpox: Acyclovir capsules are indicated for the treatment of chickenpox (varicella).
What it is
DESCRIPTION Acyclovir is a synthetic nucleoside analogue active against herpesviruses. Acyclovir capsules are formulation for oral administration. Acyclovir is a white or almost white, crystalline powder with the molecular formula C 8 H 11 N 5 O 3 and a molecular weight of 225.20.
Boxed warning
None in the current label
Causality screen of the 39 strongest signals
What the badges mean
- Labeled (related wording) The label describes this effect under different wording (e.g. 'neutropenia' for 'neutrophil count decreased'). Treated as known.
- Likely indication confounding The term appears in the label's indications: reports probably describe the disease being treated (or its progression), not a drug effect. Classic confounding by indication.
- Reporting / coding artifact An administrative, monitoring or exposure term (product issue, off-label use, 'test normal', overdose...). Not a biological reaction; a high PRR here reflects how reports are coded.
- Lab value / disease marker A laboratory result, investigation or disease-activity marker (antibody positive, CRP abnormal, swollen joint count) not described in the label. These usually mirror the monitored disease or testing protocol rather than a drug effect; not screened further.
- Not in label — unassessed Not found in the label text, not an artifact pattern. This is a genuine candidate for a formal causality assessment (Bradford Hill / WHO-UMC); it has NOT had one here.
Strongest disproportionality signals
Flagged terms (PRR ≥ 2, χ² ≥ 4, n ≥ 10), ranked by PRR; infinite PRRs (no comparator reports) excluded. The EBGM column is a second, shrinkage-based estimate (MGPS) with its 5th percentile EB05 and the BCPNN IC025; a pair flagged by PRR alone is marked PRR-only and should be read as fragile. The strata column recomputes the PRR on serious reports only and flags pairs whose reports are unusually skewed by sex or come mostly from consumers rather than clinicians. 34 further terms are in the interactive explorer. Reaction links open the term's cross-drug page.
| Reaction | Reports | PRR 95% CI · ROR · χ²/p | EBGM EB05 · IC025 | Label screen | Serious-only & strata | Trial cross-check |
|---|---|---|---|---|---|---|
| Full blood count abnormal | 52 | 83.3 63.8–108.8ROR 86.9 · χ² 4,129 / p <1e-300 | 80.6 EB05 63.8 · IC025 +5.1 3/3 methods agree | Lab value / disease marker | serious PRR 74.7 · ♀ 78% (drug 57%) | no trial data cached |
| Carpal tunnel syndrome | 65 | 78.2 61.7–99.1ROR 82.4 · χ² 4,857 / p <1e-300 | 76.2 EB05 61.8 · IC025 +5.2 3/3 methods agree | Not in label — unassessed | serious PRR 66.5 · ♀ 100% (drug 57%) | no trial data cached |
| Blood test abnormal | 63 | 75.8 59.5–96.5ROR 79.8 · χ² 4,557 / p <1e-300 | 73.8 EB05 59.8 · IC025 +5.2 3/3 methods agree | Reporting / coding artifact | serious PRR 79.9 · ♀ 100% (drug 57%) | no trial data cached |
| Sleep disorder due to a general medical condition | 71 | 74.1 59.1–93.0ROR 78.6 · χ² 5,032 / p <1e-300 | 72.4 EB05 59.4 · IC025 +5.2 3/3 methods agree | Not in label — unassessed | serious PRR 101.1 · ♀ 84% (drug 57%) | no trial data cached |
| Nephropathy | 36 | 64.7 46.9–89.4ROR 66.6 · χ² 2,189 / p <1e-300 | 62.5 EB05 47.1 · IC025 +4.6 3/3 methods agree | Not in label — unassessed | serious PRR 47.5 · ♀ 97% (drug 57%) | no trial data cached |
| Sleep apnoea syndrome | 61 | 53.8 42.1–68.7ROR 56.5 · χ² 3,101 / p <1e-300 | 52.8 EB05 42.6 · IC025 +4.8 3/3 methods agree | Not in label — unassessed | serious PRR 44.1 · ♀ 100% (drug 57%) | no trial data cached |
| Fibromyalgia | 64 | 44.4 35.0–56.4ROR 46.8 · χ² 2,669 / p <1e-300 | 43.8 EB05 35.5 · IC025 +4.6 3/3 methods agree | Not in label — unassessed | serious PRR 40.3 · ♀ 100% (drug 57%) | no trial data cached |
| Hypothyroidism | 67 | 35.5 28.1–44.8ROR 37.4 · χ² 2,207 / p <1e-300 | 35.1 EB05 28.6 · IC025 +4.4 3/3 methods agree | Not in label — unassessed | serious PRR 26.9 · ♀ 97% (drug 57%) | no trial data cached |
| Loss of personal independence in daily activities | 65 | 23.0 18.2–29.2ROR 24.2 · χ² 1,348 / p 2e-295 | 22.9 EB05 18.6 · IC025 +3.9 3/3 methods agree | Not in label — unassessed | serious PRR 20.9 · ♀ 81% (drug 57%) | no trial data cached |
| Renal disorder | 62 | 22.4 17.5–28.5ROR 23.5 · χ² 1,245 / p 6e-273 | 22.2 EB05 17.9 · IC025 +3.8 3/3 methods agree | Labeled (related wording) related wording | serious PRR 18.9 · ♀ 98% (drug 57%) | no trial data cached |
| Haematotoxicity | 12 | 21.8 12.4–38.3ROR 22.0 · χ² 218 / p 2e-49 | 20.9 EB05 12.3 · IC025 +2.6 3/3 methods agree | Not in label — unassessed | serious PRR 15.5 | no trial data cached |
| Cytomegalovirus infection | 20 | 20.2 13.1–31.3ROR 20.6 · χ² 347 / p 1e-77 | 19.8 EB05 13.4 · IC025 +3.0 3/3 methods agree | Not in label — unassessed | serious PRR 14.2 · 95% clinician | no trial data cached |
| Haemorrhage intracranial | 17 | 19.1 11.9–30.6ROR 19.3 · χ² 273 / p 1e-61 | 18.5 EB05 12.0 · IC025 +2.8 3/3 methods agree | Not in label — unassessed | serious PRR 13.2 | no trial data cached |
| Wheezing | 62 | 17.6 13.8–22.5ROR 18.5 · χ² 958 / p 1e-210 | 17.5 EB05 14.1 · IC025 +3.5 3/3 methods agree | Not in label — unassessed | serious PRR 16.5 · ♀ 80% (drug 57%) | no trial data cached |
| Therapeutic product effect incomplete | 73 | 17.4 13.9–21.7ROR 18.4 · χ² 1,113 / p 3e-244 | 17.3 EB05 14.2 · IC025 +3.6 3/3 methods agree | Reporting / coding artifact | serious PRR 22.6 · ♀ 84% (drug 57%) | no trial data cached |
| Burning sensation | 72 | 17.3 13.8–21.6ROR 18.3 · χ² 1,091 / p 2e-239 | 17.2 EB05 14.1 · IC025 +3.6 3/3 methods agree | Not in label — unassessed | serious PRR 26.2 · ♀ 100% (drug 57%) | no trial data cached |
| Lymphocyte count decreased | 17 | 16.2 10.1–25.9ROR 16.4 · χ² 227 / p 1e-51 | 15.5 EB05 9.8 · IC025 +2.7 3/3 methods agree | Lab value / disease marker | serious PRR 13.7 | no trial data cached |
| Mucosal inflammation | 25 | 16.1 10.9–23.8ROR 16.5 · χ² 340 / p 3e-76 | 15.8 EB05 11.0 · IC025 +3.0 3/3 methods agree | Labeled (related wording) related wording | serious PRR 11.9 · 95% clinician | no trial data cached |
| Clostridium difficile colitis | 10 | 15.8 8.5–29.4ROR 16.0 · χ² 125 / p 3e-29 | 14.3 EB05 7.1 · IC025 +2.1 3/3 methods agree | Not in label — unassessed | serious PRR 11.2 | no trial data cached |
| Gastrooesophageal reflux disease (acid reflux) | 69 | 14.5 11.5–18.2ROR 15.3 · χ² 855 / p 3e-188 | 14.4 EB05 11.7 · IC025 +3.3 3/3 methods agree | Not in label — unassessed | serious PRR 15.7 · ♀ 98% (drug 57%) | no trial data cached |
| Multi-organ failure | 14 | 13.4 8.0–22.6ROR 13.6 · χ² 149 / p 2e-34 | 12.2 EB05 7.0 · IC025 +2.3 3/3 methods agree | Not in label — unassessed | serious PRR 9.3 | no trial data cached |
| Cytokine release syndrome | 14 | 12.4 7.3–20.8ROR 12.5 · χ² 135 / p 2e-31 | 11.0 EB05 6.3 · IC025 +2.2 3/3 methods agree | Not in label — unassessed | serious PRR 8.6 | no trial data cached |
| Renal failure acute | 31 | 12.1 8.6–17.2ROR 12.4 · χ² 306 / p 1e-68 | 11.6 EB05 8.3 · IC025 +2.8 3/3 methods agree | Labeled (related wording) related wording | serious PRR 8.5 · ♀ 32% (drug 57%) · 93% clinician | no trial data cached |
| Multiple organ dysfunction syndrome | 20 | 12.1 7.8–18.6ROR 12.2 · χ² 192 / p 6e-44 | 11.2 EB05 7.2 · IC025 +2.5 3/3 methods agree | Not in label — unassessed | serious PRR 8.3 · 100% clinician | no trial data cached |
| Cardiac disorder | 65 | 11.6 9.2–14.8ROR 12.2 · χ² 624 / p 4e-138 | 11.4 EB05 9.2 · IC025 +3.0 3/3 methods agree | Not in label — unassessed | serious PRR 9.3 · ♀ 100% (drug 57%) | no trial data cached |
| Asthma | 75 | 11.3 9.0–14.0ROR 11.9 · χ² 694 / p 3e-153 | 11.0 EB05 9.0 · IC025 +3.0 3/3 methods agree | Not in label — unassessed | serious PRR 10.9 · ♀ 85% (drug 57%) | no trial data cached |
| Gastric ulcer | 12 | 10.6 6.0–18.6ROR 10.7 · χ² 95 / p 9e-23 | 8.9 EB05 5.0 · IC025 +2.0 3/3 methods agree | Not in label — unassessed | serious PRR 7.7 | no trial data cached |
| Febrile neutropenia | 38 | 9.7 7.1–13.2ROR 9.9 · χ² 287 / p 1e-64 | 9.1 EB05 6.8 · IC025 +2.6 3/3 methods agree | Not in label — unassessed | serious PRR 6.7 · 91% clinician | no trial data cached |
| Oral herpes | 11 | 9.5 5.3–17.1ROR 9.6 · χ² 76 / p 2e-18 | 7.7 EB05 4.3 · IC025 +1.8 3/3 methods agree | Labeled (related wording) related wording | serious PRR 14.0 | no trial data cached |
| Liver function test abnormal | 16 | 9.2 5.7–15.0ROR 9.3 · χ² 109 / p 8e-26 | 7.9 EB05 4.9 · IC025 +2.0 3/3 methods agree | Reporting / coding artifact | serious PRR 7.9 | no trial data cached |
| Intentional product use issue | 48 | 9.2 7.0–12.1ROR 9.5 · χ² 343 / p 7e-77 | 8.7 EB05 6.8 · IC025 +2.6 3/3 methods agree | Reporting / coding artifact | serious PRR 9.3 · ♀ 95% (drug 57%) | no trial data cached |
| Platelet count decreased | 56 | 8.8 6.8–11.4ROR 9.2 · χ² 384 / p 1e-85 | 8.5 EB05 6.7 · IC025 +2.6 3/3 methods agree | Labeled (related wording) related wording | serious PRR 6.2 | no trial data cached |
| Plasma cell myeloma | 20 | 8.6 5.6–13.3ROR 8.7 · χ² 127 / p 9e-30 | 7.6 EB05 5.0 · IC025 +2.1 3/3 methods agree | Not in label — unassessed | serious PRR 4.2 | no trial data cached |
| Adverse event | 39 | 7.1 5.2–9.7ROR 7.3 · χ² 200 / p 1e-45 | 6.6 EB05 5.0 · IC025 +2.2 3/3 methods agree | Reporting / coding artifact | serious PRR 10.5 · ♀ 97% (drug 57%) | no trial data cached |
| Herpes zoster (shingles) | 24 | 7.0 4.7–10.4ROR 7.1 · χ² 118 / p 8e-28 | 6.3 EB05 4.4 · IC025 +2.0 3/3 methods agree | Likely indication confounding term in indications; also in warnings | serious PRR 6.8 | no trial data cached |
| Anhedonia | 10 | 6.4 3.4–11.8ROR 6.4 · χ² 40 / p 1e-10 | 5.1 EB05 2.9 · IC025 +1.3 3/3 methods agree | Not in label — unassessed | serious PRR 4.9 | no trial data cached |
| Neutropenia (low neutrophils (infection-fighting cells)) | 46 | 5.7 4.3–7.6ROR 5.9 · χ² 175 / p 3e-40 | 5.4 EB05 4.2 · IC025 +2.0 3/3 methods agree | Not in label — unassessed | serious PRR 4.0 | no trial data cached |
| Drug exposure during pregnancy | 11 | 5.4 3.0–9.7ROR 5.4 · χ² 35 / p 2e-9 | 4.5 EB05 2.7 · IC025 +1.1 3/3 methods agree | Reporting / coding artifact | serious PRR 3.7 | no trial data cached |
| Convulsion (seizure) | 14 | 4.1 2.4–6.8ROR 4.1 · χ² 29 / p 3e-8 | 3.6 EB05 2.3 · IC025 +1.0 3/3 methods agree | Labeled (related wording) related wording | serious PRR 2.9 | no trial data cached |
Trial evidence (ClinicalTrials.gov)
Most frequently reported reactions
By raw co-report count, regardless of disproportionality. Common terms here are often the illness itself or generic outcomes.
| Reaction | Reports | PRR 95% CI · ROR · χ²/p | EBGM EB05 · IC025 | Label screen | Serious-only & strata | Trial cross-check |
|---|---|---|---|---|---|---|
| Asthma | 75 | 11.3 9.0–14.0ROR 11.9 · χ² 694 / p 3e-153 | 11.0 EB05 9.0 · IC025 +3.0 3/3 methods agree | Not in label — unassessed | serious PRR 10.9 · ♀ 85% (drug 57%) | no trial data cached |
| Therapeutic product effect incomplete | 73 | 17.4 13.9–21.7ROR 18.4 · χ² 1,113 / p 3e-244 | 17.3 EB05 14.2 · IC025 +3.6 3/3 methods agree | Reporting / coding artifact | serious PRR 22.6 · ♀ 84% (drug 57%) | no trial data cached |
| Burning sensation | 72 | 17.3 13.8–21.6ROR 18.3 · χ² 1,091 / p 2e-239 | 17.2 EB05 14.1 · IC025 +3.6 3/3 methods agree | Not in label — unassessed | serious PRR 26.2 · ♀ 100% (drug 57%) | no trial data cached |
| Sleep disorder due to a general medical condition | 71 | 74.1 59.1–93.0ROR 78.6 · χ² 5,032 / p <1e-300 | 72.4 EB05 59.4 · IC025 +5.2 3/3 methods agree | Not in label — unassessed | serious PRR 101.1 · ♀ 84% (drug 57%) | no trial data cached |
| Gastrooesophageal reflux disease (acid reflux) | 69 | 14.5 11.5–18.2ROR 15.3 · χ² 855 / p 3e-188 | 14.4 EB05 11.7 · IC025 +3.3 3/3 methods agree | Not in label — unassessed | serious PRR 15.7 · ♀ 98% (drug 57%) | no trial data cached |
| Hypothyroidism | 67 | 35.5 28.1–44.8ROR 37.4 · χ² 2,207 / p <1e-300 | 35.1 EB05 28.6 · IC025 +4.4 3/3 methods agree | Not in label — unassessed | serious PRR 26.9 · ♀ 97% (drug 57%) | no trial data cached |
| Cardiac disorder | 65 | 11.6 9.2–14.8ROR 12.2 · χ² 624 / p 4e-138 | 11.4 EB05 9.2 · IC025 +3.0 3/3 methods agree | Not in label — unassessed | serious PRR 9.3 · ♀ 100% (drug 57%) | no trial data cached |
| Loss of personal independence in daily activities | 65 | 23.0 18.2–29.2ROR 24.2 · χ² 1,348 / p 2e-295 | 22.9 EB05 18.6 · IC025 +3.9 3/3 methods agree | Not in label — unassessed | serious PRR 20.9 · ♀ 81% (drug 57%) | no trial data cached |
| Carpal tunnel syndrome | 65 | 78.2 61.7–99.1ROR 82.4 · χ² 4,857 / p <1e-300 | 76.2 EB05 61.8 · IC025 +5.2 3/3 methods agree | Not in label — unassessed | serious PRR 66.5 · ♀ 100% (drug 57%) | no trial data cached |
| Fibromyalgia | 64 | 44.4 35.0–56.4ROR 46.8 · χ² 2,669 / p <1e-300 | 43.8 EB05 35.5 · IC025 +4.6 3/3 methods agree | Not in label — unassessed | serious PRR 40.3 · ♀ 100% (drug 57%) | no trial data cached |
| Blood test abnormal | 63 | 75.8 59.5–96.5ROR 79.8 · χ² 4,557 / p <1e-300 | 73.8 EB05 59.8 · IC025 +5.2 3/3 methods agree | Reporting / coding artifact | serious PRR 79.9 · ♀ 100% (drug 57%) | no trial data cached |
| Wheezing | 62 | 17.6 13.8–22.5ROR 18.5 · χ² 958 / p 1e-210 | 17.5 EB05 14.1 · IC025 +3.5 3/3 methods agree | Not in label — unassessed | serious PRR 16.5 · ♀ 80% (drug 57%) | no trial data cached |
| Renal disorder | 62 | 22.4 17.5–28.5ROR 23.5 · χ² 1,245 / p 6e-273 | 22.2 EB05 17.9 · IC025 +3.8 3/3 methods agree | Labeled (related wording) related wording | serious PRR 18.9 · ♀ 98% (drug 57%) | no trial data cached |
| Sleep apnoea syndrome | 61 | 53.8 42.1–68.7ROR 56.5 · χ² 3,101 / p <1e-300 | 52.8 EB05 42.6 · IC025 +4.8 3/3 methods agree | Not in label — unassessed | serious PRR 44.1 · ♀ 100% (drug 57%) | no trial data cached |
| Platelet count decreased | 56 | 8.8 6.8–11.4ROR 9.2 · χ² 384 / p 1e-85 | 8.5 EB05 6.7 · IC025 +2.6 3/3 methods agree | Labeled (related wording) related wording | serious PRR 6.2 | no trial data cached |
Unlabeled candidate signals
These flagged terms were not found in the FDA label text and do not match an artifact pattern. They are the pairs that would merit a structured assessment (temporality, dose-response, biological plausibility, consistency across databases, trial or cohort evidence). The screen has not done that; string-matching misses label wording, and a strong PRR can still be stimulated reporting, co-medication, or the disease itself.
- Carpal tunnel syndrome — n=65, PRR 78.2
- Sleep disorder due to a general medical condition — n=71, PRR 74.1
- Nephropathy — n=36, PRR 64.7
- Sleep apnoea syndrome — n=61, PRR 53.8
- Fibromyalgia — n=64, PRR 44.4
- Hypothyroidism — n=67, PRR 35.5
- Loss of personal independence in daily activities — n=65, PRR 23.0
- Cytomegalovirus infection — n=20, PRR 20.2
- Wheezing — n=62, PRR 17.6
- Burning sensation — n=72, PRR 17.3
✓ A hand-written Bradford Hill causality assessment exists for this drug: Acyclovir ↔ Acute kidney injury.
Want to assess one of these properly? The runbook describes the Bradford Hill / trial-evidence workflow used on the assessment page, and handoff_top1000_signals.csv lists the strongest unassessed pairs across all drugs.
Common questions
What are the most reported side effects of Acyclovir?
In FDA FAERS, the most frequently co-reported reaction terms for Acyclovir are asthma, burning sensation, sleep disorder due to a general medical condition, gastrooesophageal reflux disease, hypothyroidism, cardiac disorder. These are report counts, not rates: FAERS has no denominator, reporting is voluntary and uneven, and a term can be reported because of the illness being treated rather than the drug.
Does Acyclovir have an FDA boxed warning?
No. The FDA label retrieved for Acyclovir has no boxed warning; see the label's Warnings and Precautions for the recognised risks.
Do FAERS signals prove that Acyclovir causes these side effects?
No. A disproportionality signal (PRR, ROR) means a reaction is reported more often with this drug than with others in the database. That can reflect a real effect, but also the disease being treated, stimulated reporting after news or litigation, co-medications, or how reports are coded. Causality needs temporality, dose-response, biological plausibility, consistency across data sources and, ideally, trial or cohort evidence. This page screens each term against the FDA label and ClinicalTrials.gov results; it does not establish causation.
Sources & methods
- FDA label via openFDA (set ID 0990b625-5dfe-4810-b5b4-ac5948599e79, effective 20260427; matched on
openfda.generic_name.exact:"ACYCLOVIR") - Bayesian shrinkage measures computed offline over all 1,317,300 cached pairs: BCPNN information component (IC, with Norén closed-form IC025) and MGPS empirical-Bayes geometric mean (EBGM, EB05) with a two-component gamma prior fitted by maximum marginal likelihood (α₁=0.57, β₁=0.02, α₂=1.42, β₂=0.70, w=0.17); unstratified
- FAERS pair statistics from the OpenPV build of 2026-09-14 (openFDA
count=aggregation, fieldpatient.drug.openfda.generic_name.exact, 73 reaction terms, min 3 co-reports); PRR/ROR with 95% CI, chi-square with Yates correction - Serious-only and stratified counts (serious:1, patient.patientsex, primarysource.qualification) from openFDA count queries; serious-only PRR uses the drug serious report total and the term global serious total (N_serious=11,882,979); strata shown only when n ≥ 20
- Seriousness and sex counts from openFDA
count=serious,patient.patientsex,seriousnessdeath
Page generated 2026-10-08. Label and PubChem text are retrieved automatically and may lag the current label; always check the current prescribing information. OpenPV is a beta research tool of the Open Source Medicine Foundation.