Diphenhydramine

Limits on what this shows

• Key null findings: No cytotoxicity at any concentration tested in beating murine cardiomyocytes [18]. No apoptotic effect in the two prostate lines [14]. Normal mononuclear cells were not driven to apoptosis [11].
• At-risk subgroups: None identifiable from the evidence. Overdose populations are the only setting where high tissue exposure occurs, and no mitochondrial measurement was found there.

Regulatory status

The injectable diphenhydramine label read [1] and the StatPearls monograph [3] do not mention mitochondrial effects.

Population evidence

No human data. Tier B only. Human T-cell leukemia lines CCRF-CEM and Jurkat: dose- and time-dependent apoptosis with cytochrome c release into the cytosol, partly reduced by Bcl-2 overexpression; peripheral blood mononuclear cells were inhibited but not apoptotic. Concentration not given in the abstract [11]. Melanoma: diphenhydramine was one of four H1 antagonists that induced apoptosis in four melanoma lines; the membrane-potential and cytochrome c readouts were reported for terfenadine, not diphenhydramine [12]. Prostate cancer lines PC-3 and DU-145: little effect from diphenhydramine [14]. Rat traumatic brain injury: the authors conclude diphenhydramine reduced mitochondrial apoptosis, although the abstract text also reports Bax up and Bcl-2 down in the diphenhydramine group, which looks pro-apoptotic and could not be reconciled without the full text [9]. Mouse brain endothelial cells in a rotenone model: diphenhydramine at 1 and 10 micromol/L (used as an H1 antagonist) reversed the protective effect of carnosine on membrane potential; diphenhydramine alone was not reported [13]. Isolated rat-lung mitochondrial MAO: diphenhydramine was a competitive inhibitor [16]. A 1964 isolated liver mitochondria swelling study of chlorpromazine, promethazine and diphenhydramine [17] was seen as a title only; its result was not read.

Mechanism

No mechanism of mitochondrial injury at therapeutic plasma concentrations is established. Cytochrome c release in leukemia lines is a cell-death phenotype in transformed cells, not a measure of mitochondrial burden in healthy tissue.

Counter-evidence held on record

Counter-evidence and limits Concentrations were not reported in any abstract read, so relevance to plasma levels in people cannot be judged. Abstracts were read through a PubMed listing summarized by a fetch tool, not in full text. The rat TBI abstract is internally inconsistent as summarized. No nutrient is implicated by this entry, and the drug's own cell-level toxicity is not a nutrient deficiency (Hard Rule 2). Framing caution Do not present this as mitochondrial damage in people taking diphenhydramine. It is a low-confidence, mixed-direction laboratory signal.

Limits on what this shows

• Key null findings: No human study measuring MDA, glutathione, total antioxidant capacity or antioxidant enzymes in diphenhydramine users was found.
• At-risk subgroups: None identified.

Regulatory status

The label [1] and StatPearls [3] are silent on oxidative stress.

Population evidence

Tier B. Rat traumatic brain injury: dose-dependent reduction in cerebral edema, neuronal degeneration, TNF-alpha and IL-1beta, and oxidative stress; MDA, SOD and glutathione peroxidase were the markers named, with direction per marker not stated in the abstract; animal numbers, doses and route not stated [9]. Cisplatin kidney injury: in cells and mice diphenhydramine reduced kidney damage markers, oxidative stress, apoptosis and inflammation without reducing cisplatin's antitumor effect; a small retrospective patient cohort showed acute kidney injury in 6.1% of prior diphenhydramine users versus 22.4% of non-users (cohort size, adjustment and CI not seen) [10]. That human figure is a clinical outcome, not an antioxidant measurement. Crayfish exposed for 96 hours to environmental diphenhydramine concentrations showed significant changes in gill and hepatopancreas antioxidant enzymes, glutathione peroxidase and glutathione S-transferase most affected (direction not stated in the summary read); this is an ecotoxicology result with no human transfer [19].

Mechanism

Not established. The rat and mouse papers attribute protection to reduced oxidative stress and inflammation but name no molecular mechanism that could be checked here.

Counter-evidence held on record

Counter-evidence and limits Abstract-level reads only (PubMed listing summarized by a fetch tool; the Kidney International record in OpenAlex carried no abstract text; the Karger page for [9] was paywalled beyond the abstract). Animal injury models at unstated doses do not show a protective effect in healthy people. Framing caution This is a protective-direction laboratory finding, not a deficiency and not a reason to take diphenhydramine or any antioxidant.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C source supports even a yellow copper grade, and no circulating claim was traced to rebut. The DAO result is context only. This is a disclosed gap, not a finding of safety. Probably a genuine literature gap, with the 429 refusals leaving some tooling limitation.

What was searched

PubMed listing pages: "diphenhydramine copper" (429 rate limit, not refetched, as the tool asked), "diphenhydramine serum copper ceruloplasmin" (429, not refetched), "diphenhydramine copper levels" (5 records). Of those five: a 2017 case report of drug-induced liver injury in a man taking up to 400 mg nightly, whose biopsy showed no iron or copper deposition and Wilson disease was excluded (a tissue finding, not a copper status measurement) [22]; the 1998 diamine oxidase study, in which diphenhydramine raised human placental DAO activity by about 19% in vitro (DAO is a copper-containing enzyme, but enzyme activity is not copper status, and the direction is the opposite of inhibition) [21]; and three records with only index-term overlap. The label [1], StatPearls [3] and the HSDB/PubChem metabolism and toxicity excerpts [4] contain no copper text.

Channel outcomes

PubMed listing: two 429 refusals and one success. OpenAlex single record for [21]: no abstract text; the abstract was read from the Springer page. No consumer depletion page naming copper for diphenhydramine could be opened (Antidote Research diphenhydramine URLs: 404 in three forms). Mixed result: a clean null on the pages that opened plus tooling-limited coverage.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C evidence of an iron effect, and no circulating claim traced. The restless legs link runs from iron status to symptom, not from drug to iron (Hard Rule 4). Disclosed gap, not a finding of safety. Probably a genuine literature gap.

What was searched

PubMed listing pages: "diphenhydramine iron absorption" (2 records, neither relevant: an activated charcoal consensus statement and an analytical chemistry method), "diphenhydramine ferritin" (5 records), "diphenhydramine hemolytic anemia" (7 records), plus the copper listing above. The ferritin hits were a JAMA 2026 restless legs syndrome review that lists diphenhydramine among centrally acting H1 antihistamines to avoid and advises iron when ferritin is 100 ng/mL or lower (abstract-level, via summarizer; this is iron deficiency worsening a symptom the drug can aggravate, the reverse of drug-caused depletion) [24], and treatment series that used diphenhydramine only as premedication [33]. Label [1], StatPearls [3] and HSDB excerpts [4] contain no iron text.

Channel outcomes

PubMed listing: three successes, no refusals on these queries. No consumer page for iron and diphenhydramine could be opened. Hematologic adverse reactions are handled in the oxygen delivery entry and are not iron data (Hard Rule 7).

Limits on what this shows

• Key null findings: A PubMed search "diphenhydramine gut microbiota" returned four records, none testing diphenhydramine on gut bacteria (one found diphenhydramine as a metabolite in crocodile gut bacteria cultures, which concerns production, not effect) [30].

Regulatory status

Label and StatPearls list constipation and other anticholinergic effects [1][3]; neither mentions the microbiome.

Cohort test

FAILED for this drug. The in vitro study [27] tested fexofenadine, cyproheptadine and desloratadine on four gut bacterial species at intestinal concentrations. Diphenhydramine was not in it. Cyproheptadine and desloratadine suppressed growth; fexofenadine promoted growth and short-chain fatty acid production (sample size not reported). Because diphenhydramine was absent, this is Tier C and a CLASS EXTRAPOLATION. Two urticaria microbiome papers found in the same search compared patients by antihistamine response and do not test a drug effect, and diphenhydramine was not mentioned [28].

Screen of marketed drugs

The Maier 2018 Nature screen (more than 1,000 marketed drugs against 40 gut strains; 24% of drugs with human targets inhibited at least one strain) was opened as a paywalled preview. Its visible text does not name diphenhydramine or any first-generation H1 antihistamine, and the per-drug results sit in supplementary tables that were not seen, so the screen neither supports nor refutes an effect [29].

Mechanism

Not established for this drug. A hypothetical route is slowed gut transit from antimuscarinic action (constipation is a labeled effect), which was not tested in any microbiome study retrieved.

Counter-evidence held on record

Counter-evidence and limits Abstracts only, read through a summarizing fetch tool. First-generation and second-generation H1 antihistamines differ in structure, and the class data are mixed in direction. Framing caution Not a claim of dysbiosis. Do not carry the cyproheptadine or desloratadine findings onto diphenhydramine.

Limits on what this shows

• Key null findings: HSDB human toxicity excerpts (read through PubChem) contain no methemoglobinemia or hemolysis text [4], and StatPearls does not mention methemoglobinemia [3].
• At-risk subgroups: Overdose and polypharmacy ingestion are the only context with a case. G6PD status was not examined in any record read.

Regulatory status

The diphenhydramine injection label lists hemolytic anemia, thrombocytopenia and agranulocytosis under hematologic adverse reactions, with no frequency [1]. StatPearls lists anemia (hemolytic), thrombocytopenia, agranulocytosis, leukopenia and pancytopenia as severe adverse effects [3]. Neither source reports a hemoglobin or red-cell measurement.

Population evidence

One case report, Tier B and confounded: a 46-year-old man ingested multiple drugs including 30 diphenhydramine pills, developed cyanosis with SpO2 85% and chocolate-colored blood with a normal PaO2, and recovered within about an hour of IV methylene blue at 1 mg/kg [20]. Other ingested drugs were not excluded as the cause, so diphenhydramine is not shown to be the culprit. A PubMed search "diphenhydramine hemolytic anemia" returned seven records and none showed diphenhydramine causing hemolysis; one 1973 G6PD jaundice record listed dimenhydrinate and had no abstract [31].

Mechanism

None named for hemolysis or methemoglobin formation. Not an iron, copper, B12 or folate pathway.

Counter-evidence held on record

Counter-evidence and limits Hard Rule 2 applies: an adverse drug reaction or overdose effect is not a nutrient deficiency. The label statement establishes that the event is reported, not how often or whether diphenhydramine caused it in any one patient. Framing caution Context flag only. It must not be shown as a deficiency or as a cause of low energy or low oxygen in ordinary users.

Limits on what this shows

• Key null findings: A PubMed search "diphenhydramine ATP" returned ten records and none measured ATP, respiration or membrane potential in diphenhydramine exposure [32].

Regulatory status

Not addressed by the label [1] or StatPearls [3].

Population evidence

Tier B, one record, abstract only: in a 1976 canine charcoal pulmonary embolism model, mitochondria isolated from heart ventricles after in vivo dosing showed oxidative phosphorylation uncoupling (measured polarographically), and diphenhydramine reduced that uncoupling while histamine enhanced it; dose and animal numbers not stated [15]. Beating murine cardiomyocytes arrested reversibly with diphenhydramine without cytotoxicity; ATP, membrane potential and respiration were not measured [18].

Mechanism

Not established. The abstract contrasts diphenhydramine with histamine, which enhanced the uncoupling; no direct action on the respiratory chain is claimed.

Counter-evidence held on record

Counter-evidence and limits This is the weakest yellow in the dossier, resting on one 1976 abstract. It shares its isolated-mitochondria search with the mitochondrial burden entry (see overlap map). Framing caution Not evidence of energy deficit or of an energy benefit in people.