Famciclovir

Direction

Neutral to possibly protective (reverse-direction finding). Not a deficiency and not a burden demonstrated in any human or animal tissue.

Regulatory status

The label sections read contain no mitochondrial statement. Nonclinical items are not mitochondrial: penciclovir was positive in the mouse lymphoma assay and caused chromosomal aberrations in human lymphocytes in vitro, and testicular toxicity occurred in rats, mice and dogs [1].

Population evidence

No human study measured mtDNA, lactate, respiration or mitochondrial enzymes in famciclovir users. PubMed "famciclovir mitochondrial" returned 3 records (the review [7], the platelet study [6] and a hepatitis B nucleoside analogue review in J Hepatol 1995 that was not opened); "penciclovir AND (mitochondrial OR mitochondria OR polymerase gamma OR mtDNA)" returned 3 records, none a mitochondrial toxicity study of penciclovir (they concern ion trapping of acidic drugs, tenofovir against Epstein-Barr virus, and a Drosophila kinase); all read as listing summaries only.

Review-level statement (Tier C)

Honkoop et al., Drug Safety 1997 [7] (a review of the fialuridine trial): per the PubMed listing summary, zidovudine, didanosine and zalcitabine have shown mitochondrial injury, while lamivudine and famciclovir "have not shown it to date"; the review recommends mitochondrial function testing for nucleoside analogues. The OpenAlex abstract field is null, so this is the listing text through a summarizer, not a reading of the paper.

Experimental data (Tier B)

Ming et al., Cardiovasc Toxicol 2025 [6]: famciclovir identified by computer-aided screening and docking as a possible aryl hydrocarbon receptor (AhR) inhibitor; in thrombin-stimulated platelets it "reduced" AhR expression, platelet activation, mitochondrial damage and oxidative stress; oral famciclovir inhibited thrombin-induced platelet aggregation and reduced FeCl3 carotid and middle cerebral artery occlusion thrombosis in animals without affecting coagulation or thrombolysis factors. The abstract names no species, doses or numbers, and mitochondrial damage is mentioned only qualitatively [6].

Related hepatic context

LiverTox (PubMed listing summary) describes famciclovir with a low rate of mild to moderate ALT elevations and no clinically apparent liver injury [19]; the label shows ALT elevation in 3.2% versus 1.5% on placebo [1]. Neither is a mitochondrial measurement.

Cohort test

Famciclovir was the studied or named drug in all of the above. The review [7] is a narrative review without drug-level stratification (Tier C); the platelet study [6] is Tier B.

Counter-evidence held on record

Counter-evidence and confounding The platelet result is an exploratory pharmacology paper with qualitative statements and cannot be taken as a nutrient or benefit claim. The review statement predates later work and says "to date". The absence of reported mitochondrial injury is not a measurement that none occurs. Framing caution This entry does not mean famciclovir protects mitochondria and does not imply any mitochondrial supplement. It records that no harm signal was found.

Direction

Reverse (possible reduction of oxidative stress). Not a nutrient deficiency; no antioxidant supplement is implied.

Regulatory status

No oxidative-stress statement in the label text read [1].

Population evidence

No human study of MDA, glutathione, SOD, total antioxidant capacity or isoprostanes in famciclovir users was found. PubMed "famciclovir AND (oxidative stress OR antioxidant OR malondialdehyde OR glutathione)" returned 11 records (10 read): the platelet study [6], an analytical-chemistry paper on famciclovir degrading in oxidizing conditions (chemical stability, not biological redox; listing only), herbal-product retrospective reviews, aldehyde oxidase pharmacology papers, the uric acid study [11], and case reports.

Experimental data (Tier B)

Ming 2025 [6]: famciclovir reduced oxidative stress and mitochondrial damage in thrombin-stimulated platelets; no ROS, MDA or other values appear in the abstract [6]. Shared with the Mitochondrial burden entry (see Overlap map).

Class-level context (CLASS EXTRAPOLATION)

Magagnoli et al., Clin Transl Sci 2026 [11]: retrospective Merative MarketScan and US Veterans Health Administration data comparing users of "cyclovir" antivirals (acyclovir, valacyclovir, famciclovir) with untreated HSV controls; serum uric acid about 1 year after the index date was lower by an adjusted 0.87 mg/dL in both cohorts (MarketScan probability-weighted, patients with baseline uric acid 6.8 mg/dL or more: 95% CI -1.59 to -0.14, p = 0.02; Veterans Health Administration: 95% CI -1.43 to -0.30, p = 0.003); the authors hypothesize guanine deaminase substrate competition. Famciclovir was named in the exposure group but the abstract gives no famciclovir-specific estimate, so by the cohort test this is class extrapolation for famciclovir (famciclovir in the cohort, not stratified), capped at yellow, and uric acid is an outcome marker and not a nutrient or a redox measurement (Hard Rule 2) [11].

Enzyme context

Xanthine oxidase does not activate famciclovir in human liver; aldehyde oxidase does [4][5]. Allopurinol (a xanthine oxidase inhibitor) did not change famciclovir pharmacokinetics in 12 healthy men (PubMed listing summary) [17].

Counter-evidence held on record

Counter-evidence and confounding The platelet paper is mechanism-screening work and not a measure of systemic redox status; the uric acid study is class-level, outcome-oriented and observational (confounding by indication and by HSV disease). Framing caution Not a vitamin or mineral finding. It is not evidence that famciclovir is an antioxidant or that an antioxidant supplement is useful.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C evidence supports even a yellow grade, and no circulating consumer claim about famciclovir and copper was found to rebut. This is a disclosed gap, not a finding of safety. Kept independent of Iron (Hard Rule 7).

What was searched

PubMed "famciclovir AND (copper OR iron OR zinc OR ferritin)": 1 record (the plasmid study [8], which uses zinc acetate only as an experimental RecA inhibitor and is not copper status evidence). PubMed "famciclovir AND (cobalamin OR magnesium OR calcium OR selenium)": 4 records, none relevant. PubMed "famciclovir AND (vitamin OR folate OR homocysteine OR nutrient OR deficiency)": 20 results, first 10 read, none a copper measurement. The label text read does not mention copper [1]; consumer pages opened state no nutrient interaction [12][13].

Channel outcomes

PubMed listing pages worked intermittently, with repeated HTTP 429 responses and several queries returning the PubMed home page instead of results (repeated once after a wait). PMC and PubMed article pages returned bot checks. Europe PMC returned 429; OpenAlex keyword search errored (single-record lookups worked); WebSearch had exhausted its shared budget. Repair pass 2026-10-09 (alternate name): PubMed "penciclovir AND (copper OR iron OR zinc OR molybdenum)" returned 3 records, read through a summarizing fetch tool: two herpes labialis treatment reviews (BMJ Clinical Evidence 2006 PMID 19454101; Cochrane 2026 PMID 42813499, topical penciclovir and zinc oxide cream as separate cold-sore treatments, no copper status measure) and the aldehyde oxidase enzymology paper [5]; none measured copper in penciclovir or famciclovir users. A repeat WebSearch call returned "budget used up" and no search ran. Genuine gap; the remaining tooling limit is that only listing summaries were read and the famciclovir-only phrasings "copper OR ceruloplasmin" were not run.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C source for iron status and no circulating claim to rebut. Not a finding of safety. Kept independent of Copper (Hard Rule 7).

What was searched

The same PubMed mineral and vitamin queries as Copper (no iron measurement in any record read). The label text read does not mention iron; its anemia figure (0.1% versus 0.0%) is a laboratory abnormality and not an iron-status measure (Hard Rule 2) [1]. Context only: human aldehyde oxidase, the enzyme that activates the 6-deoxy intermediate, carries two 2Fe-2S clusters as cofactors (Tier D reference page) [14]; no source relates this to iron status or to famciclovir dosing.

Channel outcomes

Same as Copper, including the repair-pass penciclovir mineral query (3 records, none an iron measurement). The hematologic query (famciclovir AND hemolytic, methemoglobinemia, neutropenia, thrombocytopenia: 13 records) contained no iron-deficiency, ferritin or iron-absorption record. Repair pass: PubMed "penciclovir AND (anemia OR hemolysis OR methemoglobin OR hemoglobin OR neutropenia)" returned 1 record, DeBellis et al., Blood Cells Mol Dis 2003 (PMID 12972037), an in vitro sickling study of purine antivirals in which penciclovir appears only as an indexing keyword; OpenAlex single-record lookup shows a null abstract and no mention of famciclovir, so it is not iron evidence and was not used. Genuine gap with a tooling-limited component (ferritin or iron-absorption phrasings were not rerun on the penciclovir name).

Direction

Possible increase in antibiotic-resistance gene transfer potential (a microbiome risk), and a composition shift in a non-gut site. Not a nutrient deficiency; no supplementation implication.

Regulatory status

The label text read contains no microbiome, dysbiosis or Clostridioides difficile statement. After a radiolabeled oral dose 73% of radioactivity was recovered in urine and 27% in feces over 72 hours [1].

Population evidence

No human gut microbiome study with famciclovir found. PubMed "famciclovir AND (microbiome OR microbiota OR dysbiosis OR Clostridioides OR Clostridium)" returned 3 records: the plasmid study [8], a Medical Letter sexually transmitted infection drug table, and the cat study [9].

Mechanistic data (Tier B)

Alav et al., npj Antimicrobials and Resistance 2026 [8]: 14 nucleoside analogues screened for effect on conjugation of GFP-tagged plasmids. Famciclovir increased transfer of the IncK ESBL plasmid pCTgfp in E. coli EC958 at 1, 10 and 100 ug/mL in the screen (the text is inconsistent on which doses were significant: the dose-response section says only 100 ug/mL); no fold change is given in the text (figure only); growth at 100 ug/mL was not different from the DMSO control; famciclovir was not tested in K. pneumoniae; the authors identified no mechanism for famciclovir [8]. The label penciclovir Cmax after a 500 mg dose is 3.3 mcg/mL, so 100 ug/mL is about 30 times higher and 1 ug/mL is below it [1][8].

Animal data (Tier B, non-gut)

Lucyshyn et al., J Feline Med Surg 2021 [9]: shelter cats, famciclovir about 30 or 90 mg/kg orally every 12 hours for 7 days, placebo or untreated; 124 conjunctival swabs analyzed by 16S sequencing. Beta diversity (community membership and structure) differed between day 1 and day 8 in all famciclovir-treated cats regardless of health status or dose; alpha diversity did not change in any subgroup; the authors attribute the main shift to time in the shelter and say famciclovir "may affect" beta diversity [9]. A conjunctival swab is not a gut sample.

Cohort test

Famciclovir was in both studies (Tier B). In the same plasmid study acyclovir and valacyclovir raised transfer in K. pneumoniae; those are related-drug results and were not applied to famciclovir.

Counter-evidence held on record

Counter-evidence and confounding The plasmid effect is a laboratory conjugation assay in one E. coli strain at concentrations that include 30 times the human peak plasma value; colonic drug concentrations in people were not measured and a beta-diversity change in cat eyes is not a gut finding. The cat study cannot separate drug from time in the shelter. Framing caution Not a vitamin or mineral finding, and not evidence that famciclovir harms the human gut microbiome. No probiotic or supplement is implied.

Limits on what this shows

• Null and contradicting findings: PubMed "famciclovir anemia" returned 2 records, neither about anemia (an Epstein-Barr virus antiviral review and a dapsone erythema multiforme chart review). PubMed "famciclovir AND (hemolytic OR methemoglobinemia OR neutropenia OR thrombocytopenia)" returned 13 records, 10 read: veterinary elephant, dog and cat reports, antiviral reviews and the TTP case; no human hemoglobin or hemolysis series. PubMed "famciclovir AND (thrombotic OR microangiopathy OR purpura OR hemolytic uremic)" returned 6 records: the platelet study [6], the FAERS analysis [20], a kitten trial, one case of famciclovir-attributed leukocytoclastic vasculitis (a skin vasculitis, listing summary only, not hematologic), the TTP case [10], and a valaciclovir review (a different drug, not applied). No second famciclovir TTP report or microangiopathy cohort was found. The label laboratory table shows anemia at placebo-like frequency [1]. Repair pass 2026-10-09 with the alternate name: PubMed "penciclovir AND (anemia OR hemolysis OR methemoglobin OR hemoglobin OR neutropenia)" returned 1 record, an in vitro sickling study of purine antivirals (PMID 12972037) in which penciclovir is an indexing keyword only, famciclovir is absent and the abstract is null; it was not used.
• At-risk subgroups: Not defined by any source read. Renal impairment raises penciclovir exposure (label), but no hematologic signal is tied to it [1].

Direction

Possible drug-toxicity outcome (microangiopathic blood injury). It is not a nutrient deficiency, not iron or B12 deficiency, and nothing here supports supplementation. Hard Rule 2: anemia and TTP are outcomes, not nutrients.

Regulatory status

Label Table 3 (genital herpes suppression; famciclovir n = 660, placebo n = 210): anemia 0.1% versus 0.0%, leukopenia 1.3% versus 0.9%, neutropenia 3.2% versus 1.5% [1]. Postmarketing list includes thrombocytopenia; the label text read does not mention TTP, hemolysis or methemoglobinemia [1]. Thresholds behind the laboratory percentages were not extracted.

Population evidence

Case report (Tier B, one patient): Ryz KS, Klassen J, Gough JC, Ahmed SB. "Famciclovir and development of thrombotic thrombocytopenic purpura." Ther Apher Dial 2007;11(6):458-459 [10]. Only the title and bibliographic record were read: the OpenAlex abstract field is null and the PubMed article page returned a bot check, so the patient details, dose, platelet count, hemoglobin and outcome are not known to this review. Pharmacovigilance (Tier A for the cohort, signal detection only): Fang et al., Sci Rep 2024 [20] mined 17,652,186 FDA Adverse Event Reporting System reports and identified 432 famciclovir-related adverse events (the article text also gives 1,317 and 1,371 counts for other items without explanation); purpura (n = 4) had ROR 18.23 (confidence interval not stated), PRR 18.18, IC 4.18 and EBGM 18.17 and the authors state it is not in the FDA labeling; pancytopenia is listed among unlabeled adverse events with no count or statistic; thrombocytopenia, anemia, neutropenia, hemolysis and the blood and lymphatic system organ class were not reported in the text read. The authors state causality cannot be established, reporting is subject to bias, and confounders were not controlled [20]. Purpura in this analysis may be a skin or vascular term and is not necessarily TTP.

Direct measurement

Hemoglobin was not reported as a continuous outcome in any famciclovir source opened. The only red-cell measure is the label anemia percentage above [1]; the pharmacovigilance study [20] reports reporting-odds statistics, not blood values.

Mechanism

None named. No pathway for famciclovir or penciclovir injuring red cells or platelets is stated in the label text read [1].

Cohort test

The label and the case report name famciclovir (Tier A for the label events, Tier B for the case). No cohort measured hemoglobin, haptoglobin, LDH or platelets in famciclovir users versus non-users. The valacyclovir and acyclovir TTP/HUS literature is a different drug and was not used (Hard Rule 6).

Why yellow, not orange

Orange needs at least one Tier A study with the drug in the cohort showing a real, bounded effect on the measured marker. The label shows no excess anemia, the FAERS analysis [20] is a spontaneous-report signal that cannot establish causality and did not measure hemoglobin or platelets, and the one case report [10] could not be read. A near-orange candidate: full tables of Fang 2024 (pancytopenia count and statistics) would sharpen it but would still be a reporting signal.

Counter-evidence held on record

Counter-evidence and confounding A single case cannot separate the drug from the underlying herpes zoster, co-medication or unrelated causes of TTP, and its details were not retrieved. Confounding by indication is presumed (Hard Rule 6). Framing caution This is a rare drug-toxicity signal at most, not an iron, B12 or folate problem. New pallor, bruising, fever, dark urine or confusion on famciclovir warrants prompt clinical assessment; stopping or changing the drug is a prescriber decision.

Direction

Possible reduction of bacterial ATP content in vitro. Not a human energy-metabolism finding, not a nutrient deficiency.

Regulatory status

No ATP or energy-metabolism statement in the label text read [1].

Population evidence

None in humans. PubMed "famciclovir AND (ATP OR respiration OR bioenergetics)" returned 6 records: the plasmid study [8], the aldehyde oxidase papers [4][5], a raloxifene aldehyde oxidase paper [18], the nucleoside-analogue mitochondrial review [7] and a trial bibliography containing the words "adenosine triphosphate" with no famciclovir data.

Experimental data (Tier B)

Alav 2026 article page [8] (read through a summarizing fetch tool): 100 ug/mL famciclovir significantly reduced ATP content in E. coli EC958/pCTgfp after 4 hours (figure panel only; no numeric values in the text). The same experiments found no significant effect of famciclovir on ROS, outer or inner membrane permeability, or membrane potential. The authors suggest nucleotide pool disruption after intracellular phosphorylation and state the study "does not provide evidence for direct causality between ATP modulation and changes in plasmid conjugation frequency" [8].

Cohort test

Famciclovir was the drug tested (Tier B). It is a bacterial system at a concentration about 30 times the label Cmax of 3.3 ug/mL for 500 mg [1]; it does not speak to human ATP synthesis.

Counter-evidence held on record

Counter-evidence and confounding Membrane potential and ROS were unchanged in the same bacterial system, so the ATP signal is not accompanied by an electron-transport or oxidative-stress signal there. The reverse-direction platelet data and the review statement in the Mitochondrial burden entry point away from mitochondrial injury. Framing caution Not a statement about human energy, fatigue or ATP. Fatigue is listed in the HIV label comparison (4% versus 2% on acyclovir) as an adverse event, not as an ATP finding [1].