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MyDeficiencies

Famotidine

histamine H2 receptor antagonist

Also sold as Pepcid, Pepcid AC.

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Findings for this medication

Research about this medication, graded for how strongly each finding can be attributed to it. Nothing here describes your own levels.

Famotidine has almost no classical depletion machinery. It is cleared mostly by the kidney (65 to 70% renal, 30 to 35% metabolic), has a single known human metabolite, inhibits no CYP enzyme other than a weak effect on CYP1A2, and its label contains no mention of vitamin B12, iron, calcium or any other nutrient [1][2][3]. The one real mechanism is the one the drug is built to produce: reduced gastric acid. Acid-dependent absorption effects are a legitimate drug-caused pathway to evaluate, and they are evaluated here, but the drug's intended acid suppression is never itself filed as a deficiency.

histamine H2 receptor antagonist · ATC A02BA03 (WHO ATC/DDD Index [4]) · also sold as Pepcid and Pepcid AC; many generics · reviewed 2026-10-04. This describes research about the medication. It does not say anything about your own levels or health.

How to read the evidence grades

These colours describe how strongly a finding can be attributed to the medication in the research. They are not a severity score and they say nothing about your own health.

Grades describe how strongly a finding can be attributed to Famotidine in the research — not how common it is, how serious it would be, and not anything about you personally.

Red — Severe
Two or more independent Tier A sources, this drug in the cohort, the nutrient directly measured, consistent direction, clinically meaningful magnitude, a named mechanism, no substantial contradicting Tier A evidence.
Orange — Moderate
At least one Tier A source with this drug in the cohort showing a real but bounded deficit: an exposure window, a subgroup, low frequency, or a functional rather than circulating deficit.
Yellow — Low
Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction.
Not graded — disclosed evidence gap
Nothing has been measured here, so this finding cannot be graded. That does not establish either safety or harm, and it is not the same as a White result.Source definition: Not gradeable under the rules above because nothing has been measured. Published as a documented gap, never as reassurance that there is no effect.

What the research found

Vitamin B12: absorption, class extrapolation, long-term use

Yellow — Low

See study details

Acid-dependent release of food-bound B12 is a plausible pathway, and H2 blocker use for 2 or more years is modestly associated with a B12 deficiency diagnosis, but no source retrieved studied famotidine by name.

Full detail and limits

What the grade means here: Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction. Yellow

Limits on what this shows

  • Null findings: None found for famotidine. No trial measuring B12 absorption on famotidine was located.
  • At-risk subgroups: Older adults, long-term continuous users, people on metformin, vegetarians and vegans (a nutrition-status context, not a famotidine finding).

Regulatory status

Not mentioned anywhere in the famotidine labels [1][2][3].

Population evidence

Lam 2013 [5]: nested case-control, Kaiser Permanente Northern California, 25,956 cases and 184,199 controls. H2 blocker supply of 2 or more years: OR 1.25 (95% CI 1.17 to 1.34). PPIs for context: OR 1.65. The paper pooled all H2 blockers; individual agents were not reported, so famotidine is not documented in the cohort. Observational study of 77 older adults on chronic acid suppression [8]: about half had B12 below 300 pg/mL, no difference by drug type, but ranitidine was the H2 blocker studied and there was no unexposed comparison group.

Mechanism

Present. Reduced gastric acid impairs pepsin-mediated release of protein-bound B12. A letter citing a cimetidine study reports food-bound B12 uptake falling from 5.3% to 2.5% (secondhand, primary not opened) [7]. A 78-year-old on cimetidine and later agents developed B12 deficiency with raised methylmalonic acid (757 nmol/L) and homocysteine (27.3 umol/L) over about 4.5 years [6]. The same letter describes a patient on famotidine with B12 deficiency, but metformin was a competing cause [7].

Class cohort test

Class extrapolation from H2 blockers to famotidine. Passes on mechanism: the B12 pathway depends on acid suppression, which famotidine produces [1]. It does not depend on cimetidine-specific properties (CYP inhibition, antiandrogen effects). PPI findings are not borrowed; the omeprazole malabsorption trial [9] was read for context only.

Counter-evidence held on record

Counter-evidence and confounding Confounding by indication is presumed: patients with chronic reflux, older age and metformin co-use have higher baseline B12 deficiency. Lam 2013 excluded people with under 2 years of exposure and association weakened after stopping, but this does not exclude residual confounding. The effect size is modest. Framing caution This is a monitoring prompt for long-term users, not a deficiency claim for famotidine. Do not recommend B12 supplementation from this entry.

Magnesium: single famotidine case report, with contrary switch data

Yellow — Low

See study details

One case of hypomagnesemia with hypocalcemia after 2 years of famotidine has been published, while 11 patients with PPI-associated hypomagnesemia had no recurrence on famotidine.

Full detail and limits

What the grade means here: Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction. Yellow

Limits on what this shows

  • Null findings: Systematic review of 64 patients with PPI-associated hypomagnesemia [21]: after switching to an H2 blocker, hypomagnesemia did not recur in 32 patients (ranitidine 17, famotidine 11, cimetidine 4), while it recurred on switching between PPIs. This is a directional contrast, not a controlled test.
  • At-risk subgroups: Not defined from one case. Age, diuretic use and renal disease are generic hypomagnesemia contexts, not famotidine findings.

Regulatory status

Not listed in the famotidine labels [1][2][3].

Population evidence

Case report [20]: 55-year-old woman, famotidine 20 mg twice daily for 2 years, magnesium 0.55 mg/dL, calcium 5.7 mg/dL, low PTH. Normalized within weeks of stopping. No prior PPI documented. The authors call it the first such case and propose impaired acid-dependent magnesium absorption but acknowledge uncertainty. This is Tier B, drug-specific.

Mechanism

Proposed but not demonstrated for famotidine. The hypocalcemia in the case is secondary to hypomagnesemia, which suppresses PTH. This is not a calcium absorption finding.

Counter-evidence held on record

Counter-evidence The switch data argue against a reproducible famotidine effect, and a single case cannot establish causation. Causal assessment (dechallenge) was positive in the case but rechallenge was not reported. Framing caution Theoretical only. Do not present as an expected effect, and do not recommend magnesium supplementation. Supplemental magnesium has an NIH upper limit for supplements, and none is recommended here.

The five body-system areas

Each finding is counted once, in a single area. Where a study is mentioned in a second area it is context only.

Mitochondrial Burden

Each finding below is counted once, in this area only.

Mitochondrial burden

Not graded — disclosed evidence gap

See study details

No famotidine-specific or defensible class-level evidence on mitochondrial structure, electron transport or biogenesis was retrieved.

Full detail and limits

What the grade means here: Nothing has been measured here, so this finding cannot be graded. That does not establish either safety or harm, and it is not the same as a White result. Ungraded

Limits on what this shows

  • Why no grade: Dual condition met: no Tier B or C source was opened to support even a yellow grade, and no circulating consumer claim of famotidine mitochondrial damage was found to rebut. This is a disclosed gap, not a finding of safety. Re-check when PubMed access is restored.

What was searched

Two phrasings via web search ("famotidine mitochondria mitochondrial toxicity respiration in vitro hepatocytes"; "famotidine ATP depletion OR mitochondrial membrane potential OR oxidative phosphorylation cells H2 antagonist cytotoxicity"). Results were generic mitochondrial-toxicity methods papers and none concerned famotidine. One possibly relevant record, a 1988 study of H2 blocker toxicity in isolated rat hepatocytes (PMID 2891842), could not be read because PubMed returned a bot-verification page.

Channel outcomes

PubMed eutils, Crossref, OpenAlex, Europe PMC, Semantic Scholar, api.fda.gov and doi.org: proxy refused the connection (403 on CONNECT). PubMed HTML search listing: 429 rate limit. PubMed and PMC article pages: reCAPTCHA bot-verification. DailyMed label pages: opened; no mitochondrial content. Unpaywall: not separately tested (same blocked-host class). This is a mixed result, a clean zero on open web search plus tooling-limited access to the primary databases.

Oxidative Stress

Each finding below is counted once, in this area only.

Oxidative stress: reverse direction, in vitro radical scavenging, minimal in vivo effect

Yellow — Low

See study details

Famotidine's own chemistry scavenges radicals in vitro, so a pro-oxidant or antioxidant-depleting effect is not supported; in a mouse model its in vivo effect on oxidative markers was minimal.

Full detail, limits and 1 references

What the grade means here: Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction. Yellow

Limits on what this shows

  • Null findings: Mouse GSH and GSSG unchanged by famotidine alone [17].

Regulatory status

No oxidative-stress warning in the famotidine labels [1][2][3].

Population evidence

None in humans. Biedrzycki 2023 [16]: in vitro comparison of ranitidine, cimetidine and famotidine; famotidine showed 20 to 70% scavenging across superoxide, hydroxyl, nitric oxide, hydrogen peroxide, ABTS and DPPH assays, moderate overall and weaker than ranitidine, with no more than modest anti-glycation effects. Gholami 2025 [17]: 40 male mice, famotidine 0.75, 1.5 or 3 mg/kg orally twice daily for 3 days before gamma irradiation; famotidine alone did not significantly lower lipid peroxidation at 2 Gy, showed some effect at 4 Gy, and showed no consistent change in GSH or GSSG.

Mechanism

Direct radical scavenging by the drug molecule, shown in vitro. Not shown to alter systemic antioxidant capacity in humans.

Counter-evidence held on record

Counter-evidence Neither source measured oxidative stress in human famotidine users. The mouse model was a radiation study, not chronic dosing. Framing caution This is not an antioxidant benefit claim and not a deficiency claim. Famotidine should not be described as depleting antioxidants.

Copper and Iron Balance

Copper and iron are assessed separately. Nothing here is counted twice.

Iron: absorption, class extrapolation, 2 or more years

Yellow — Low

See study details

Non-heme iron absorption depends partly on gastric acid, and long-term H2 blocker use is associated with an iron deficiency diagnosis in a large class-level dataset that did not name famotidine.

Full detail and limits

What the grade means here: Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction. Yellow

Limits on what this shows

  • Null findings: None located for famotidine.
  • At-risk subgroups: Menstruating women, people with low dietary iron, people with GI blood loss, concurrent PPI users.

Regulatory status

Not mentioned in the famotidine labels [1][2][3].

Population evidence

Lam 2017 [10]: community-based case-control, 77,046 incident iron deficiency cases and 389,314 controls (1999 to 2013). H2 blocker users without concurrent PPI use, 2 or more years: adjusted OR 1.58 (95% CI 1.46 to 1.71). Individual H2 blockers were not named. A narrative review of H2 blocker hematologic effects lists reduced iron absorption as one of four categories of harm [11].

Mechanism

Present but unmeasured for famotidine. Acid solubilizes and reduces dietary non-heme iron. No famotidine iron-absorption study was opened.

Class cohort test

Class extrapolation from pooled H2 blockers. The mechanism (acid-dependent absorption) is shared across H2 blockers, so the extrapolation is defensible at yellow only. PPI iron data were not borrowed.

Counter-evidence held on record

Counter-evidence and confounding The diagnosis of iron deficiency is itself a trigger for GI work-up, and GI bleeding from the underlying acid disease could cause iron deficiency independent of the drug (confounding by indication). The study cannot separate these. Framing caution Iron deficiency is not anemia, and anemia is not iron deficiency (Hard Rule 2). Iron supplementation is not recommended by this entry; excess iron carries its own risks.

Copper: in vitro chelation of Cu(II) only, no human measurement

Yellow — Low

See study details

Famotidine binds copper(II) strongly in the test tube, but no study measured copper status in famotidine users.

Full detail and limits

What the grade means here: Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction. Yellow

Limits on what this shows

  • Null findings: None located; no copper measurement in famotidine users was found.

Regulatory status

Not mentioned in the famotidine labels [1][2][3].

Population evidence

None in humans. Kubiak 1996 [18] determined a crystal structure of a copper(II)-famotidine complex and studied the ternary Cu2+-famotidine-histidine system by potentiometric and spectroscopic methods. Famotidine was "a very competitive chelating agent" even against histidine (Tier B, famotidine-specific, in vitro chemistry).

Mechanism

Chelation chemistry is demonstrated in vitro. Physiological relevance (plasma concentrations, competition from ceruloplasmin and albumin, effect on absorption or excretion) is not shown. No copper-specific search result linked acid suppression by H2 blockers to copper status.

Counter-evidence held on record

Counter-evidence In vitro binding does not show depletion; many drugs bind metals in a flask. No clinical copper deficiency report for famotidine was found. Framing caution Mechanistic curiosity only. Not a deficiency claim, and not a reason to supplement copper.

Gut Microbiome Risk

Each finding below is counted once, in this area only.

Gut microbiome risk: class extrapolation, small effect relative to PPIs

Yellow — Low

See study details

H2 blocker use as a class is associated with small or undetectable microbiome change; famotidine was not named in the retrieved studies.

Full detail and limits

What the grade means here: Mechanistic plausibility only (Tier B), or class extrapolation (Tier C) with no drug-specific human status data; also used for context flags and interactions running the other direction. Yellow

Limits on what this shows

  • Null findings: The randomized trial found no separable H2 blocker microbiome profile [14]. The large Dutch three-cohort PPI study [15] did not analyze H2 blockers at all, so it supplies no H2 blocker data and its PPI results are not borrowed.
  • At-risk subgroups: Not defined. The hemodialysis cohort is a heavily confounded population.

Regulatory status

No microbiome statement in the famotidine labels [1][2][3].

Population evidence

Randomized trial [14] (as summarized by an aggregator page; full text not opened): PPI group n=23, H2 blocker group n=26, metagenomic sequencing of oral and fecal samples. PPI use increased oral-to-gut transmission (classifier AUROC 0.924 for PPI versus controls); the H2 blocker classifier AUROC was 0.509, i.e. no discernible signature. The H2 blocker agent was not specified in the retrieved text. Cross-sectional study [13] of 193 hemodialysis patients (32 H2 blocker users, 23 PPI users, 138 non-users): H2 blocker users had enrichment of Prevotella 2, Phascolarctobacterium, Christensenellaceae R-7 group and Eubacterium oxidoreducens group; PPIs caused larger changes. Famotidine was not named.

Mechanism

Plausible but small: less gastric acid may allow more oral and upper-GI bacteria to survive transit. Not demonstrated for famotidine.

Class cohort test

Class extrapolation from H2 blockers as a pooled group. Passes only as a weak yellow because the shared mechanism (acid reduction) applies, and because the class data themselves show minimal effect.

Counter-evidence held on record

Counter-evidence The strongest class-level data point toward no detectable effect. Taxa differences in dialysis patients may reflect kidney disease, diet and medications other than the blocker. Framing caution Do not extend PPI microbiome findings to famotidine. This entry is not a claim of dysbiosis.

Oxygen and ATP Output

Oxygen delivery and ATP production are assessed separately. Nothing here is counted twice.

Oxygen delivery/utilization

Not graded — disclosed evidence gap

See study details

No evidence was found that famotidine reduces red-cell mass, hemoglobin or tissue oxygen delivery through any route independent of the iron and B12 entries.

Full detail and limits

What the grade means here: Nothing has been measured here, so this finding cannot be graded. That does not establish either safety or harm, and it is not the same as a White result. Ungraded

Limits on what this shows

  • Why no grade: Dual condition met: no direct Tier B or C evidence on oxygen delivery, and no circulating claim about oxygen-carrying capacity for famotidine was found. Any anemia effect would run through the iron or B12 entries and is not double-counted here (see Overlap map). Anemia is an outcome, not a nutrient (Hard Rule 2).

What was searched

Web search "H2 receptor antagonist famotidine hemoglobin anemia hematologic adverse effects red blood cell" and publisher fetches. The FDA label lists postmarketing agranulocytosis, pancytopenia and leukopenia [2], which are white-cell and platelet events rather than oxygen-carrying measures. A 1988 review found 88% of 85 H2 blocker cytopenia cases were with cimetidine [11]. A randomized trial of 40 mg famotidine in 60 patients on chemoradiotherapy found no significant hemoglobin difference versus placebo [19], but the population and purpose (radioprotection) make it weak context only.

Channel outcomes

Same as Mitochondrial burden: database channels blocked or bot-challenged; open web search returned no hemoglobin-lowering report for famotidine. Tooling-limited plus a likely thin literature.

ATP production

Not graded — disclosed evidence gap

See study details

No evidence on famotidine and cellular ATP synthesis or energy output was retrieved.

Full detail and limits

What the grade means here: Nothing has been measured here, so this finding cannot be graded. That does not establish either safety or harm, and it is not the same as a White result. Ungraded

Limits on what this shows

  • Why no grade: Dual condition met: no Tier B or C evidence and no circulating "low energy" claim specific to famotidine found. This is not a finding of safety. Kept independent of Mitochondrial burden and Oxygen delivery/utilization (Hard Rule 7).

What was searched

Same two web-search phrasings as Mitochondrial burden, including ATP depletion and oxidative phosphorylation terms. No famotidine result. The same hepatocyte record (PMID 2891842) could not be opened.

Channel outcomes

Same as Mitochondrial burden. Tooling-limited for database searches; clean zero on open web search.

Claims checked and not supported

These circulate online for this medication. They are listed here only so they can be answered, never as findings, and they are not graded.

A claim with no supporting evidence is not necessarily disproven. Where evidence actually argues against a claim, that is stated below.

  • Calcium deficiency and bone loss

    Where it comes from: Consumer drug-nutrient lists and retailer pages; the general acid-suppressor fracture narrative.

    Why it fails: NHANES analysis of 4,232 adults aged 60 and over, with 169 H2 blocker users (cimetidine, ranitidine, famotidine, nizatidine): no association of long-term H2 blocker use with lumbar spine BMD (adjusted beta 0.0182, 95% CI -0.0050 to 0.0413) and no difference in serum calcium [12]. Fracture and BMD are outcomes, not calcium deficiency. Limits: H2 blockers were pooled and calcium absorption was not measured. The hypocalcemia in the magnesium case report is secondary to hypomagnesemia [20].

  • Folate and vitamin A

    Where it comes from: Tier D retailer page listing famotidine with folic acid "depletion" and vitamin A "suppression" [23].

    Why it fails: No primary source is given on the page, the famotidine labels do not mention either nutrient, and no mechanism or class-level study was found in searches. Unsupported at any tier.

  • Zinc (provisional, tooling-limited)

    Where it comes from: Same Tier D page [23], which cites a 1991 study of gastric acid inhibition and zinc absorption in man, plus an unrelated cimetidine neuropathy report and a pediatric review.

    Why it fails: No famotidine zinc measurement was found. The cited 1991 study could not be opened (PubMed bot-verification; publisher 403), so it was not read or used. An acid-dependent absorption mechanism is plausible, so this is not a mechanistic rule-out and is not graded white on the merits. It is held out of publication as a deficiency claim pending a re-check that could convert it to yellow.

  • "Famotidine causes B12 deficiency" as a stated fact

    Where it comes from: Mayo Clinic FAQ and consumer pages [22].

    Why it fails: The statement is general to heartburn medicines, names no study and does not distinguish famotidine. It is carried here as yellow class extrapolation with the confounding limits shown, not as an established effect. Included so readers see the difference.

How the body handles this medication

Metabolism and enzymes
Minimal first-pass metabolism. The S-oxide is the only identified human metabolite. Famotidine is a weak CYP1A2 inhibitor; the label warns of substantially raised tizanidine levels. No CYP induction is described.[1][3] DailyMed generic label sec 7, 12.3; FDA label
Elimination
Renal 65 to 70% and metabolic 30 to 35%. 25 to 30% of an oral dose is recovered unchanged in urine. Renal clearance 250 to 450 mL/min, indicating some tubular secretion. Half-life 2.5 to 3.5 hours.[1][3]
Absorption
Incompletely absorbed; oral bioavailability 40 to 45%. Plasma protein binding 15 to 20%. The named absorption transporter is not given in the label sections retrieved.[1][3]
Renal impairment
Exposure rises at least 2-fold in moderate and at least 5-fold in severe renal impairment. Dose reduction is advised below CrCl 60 mL/min. CNS adverse reactions (confusion, delirium, seizures) are reported in the elderly and in renal impairment.[1][3]
Food, antacid and mineral interactions
Food may slightly raise and antacids slightly lower famotidine bioavailability, with no clinical consequence; it may be given with antacids. No iron, calcium, zinc or other mineral interaction is listed.[1][3]
Gastric pH effects
Reduces both acid concentration and volume of gastric secretion; onset within 1 hour. Absorption of pH-dependent drugs (dasatinib, delavirdine, cefditoren, fosamprenavir) falls and co-use is not recommended.[1][2] sec 12.2, 7

What the research says is worth watching

General information from the medication label and the studies above. It is not personal medical advice, and any change belongs with your prescriber.

  • Lab / long-term use

    Vitamin B12 status

    Worth considering serum B12, with methylmalonic acid or homocysteine if borderline, for people on continuous therapy beyond about 2 years, especially older adults and people also on metformin. This is a monitoring prompt, not a finding of famotidine-caused deficiency.

  • Lab / symptoms

    Iron indices

    Complete blood count and ferritin if fatigue, pallor or heavy menstrual loss, or if on long-term therapy with other risk factors. New iron deficiency in an adult warrants evaluation for GI blood loss rather than assumption of a drug cause.

  • Lab / unusual symptoms

    Serum magnesium and calcium

    Only if muscle cramps, tetany, arrhythmia or unexplained low calcium occur on long-term therapy, or if a diuretic or other magnesium-lowering drug is co-prescribed. The evidence is one case report [20].

  • Renal / CNS

    Kidney function

    Famotidine exposure rises at least 2-fold in moderate and 5-fold in severe renal impairment. Dose reduction below CrCl 60 mL/min and attention to confusion or lethargy are label matters [1][3].

  • Timing

    Supplement separation

    No separation interval is required by the label for antacids [1]. No mineral or vitamin supplement timing interaction is listed for famotidine. No supplement dose is recommended or appears anywhere in this dossier, so no upper-limit comparison was needed.

  • Special populations

    Pregnancy, pediatrics

    Pregnancy and pediatric nutrient considerations were not assessed from the label sections retrieved. Renal impairment and older age are the documented sensitivity groups [1][3].

Overlap map, to prevent double counting

  • Vitamin B12

    Shares evidence with: Iron

    Note: Same mechanism (acid reduction impairing food-bound micronutrient release) and the same research group and database (Kaiser Permanente Northern California case-control series, [5] and [10]). Treat as one mechanism and one data source, not two independent confirmations.

  • Vitamin B12; Iron

    Shares evidence with: Oxygen delivery/utilization (ungraded)

    Note: Any effect on hemoglobin or oxygen carrying would be mediated by these two entries. Oxygen delivery is not counted separately. The 1988 review [11] is cited in the iron entry and the oxygen-delivery entry for different statements.

  • Magnesium

    Shares evidence with: Fails verification: calcium

    Note: The calcium drop in the case report [20] is secondary to low magnesium. Both use the hypomagnesemia systematic review [21] as context. Not an independent calcium signal.

  • Oxidative stress

    Shares evidence with: Mitochondrial burden; ATP production (ungraded)

    Note: No shared evidence. Radical scavenging in vitro [16] says nothing about mitochondria or ATP, and is not used for either.

  • Mitochondrial burden

    Shares evidence with: ATP production

    Note: Both ungraded for the same reason (the same unread hepatocyte record and the same search outcomes). The gap is one gap viewed twice, not two independent gaps.

  • Copper

    Shares evidence with: Iron

    Note: No shared source. The copper evidence [18] is an in vitro chelation study only. The pair is graded separately per Hard Rule 7.

  • Gut microbiome risk

    Shares evidence with: Vitamin B12; Iron

    Note: No shared source, but a microbiome shift is a hypothetical modifier of absorption and is not claimed here.

All references used above
  1. Famotidine tablets, prescribing information (DailyMed generic label). — Regulatory label (Tier A). Sections 7, 12.2, 12.3 read; revision date shown on retrieved page as 9/2026. Source of CYP1A2, elimination, bioavailability and interaction facts.
  2. PEPCID (famotidine) tablet, DailyMed label, Bausch Health US LLC. — Regulatory label (Tier A). Revision June 1, 2024. Indications, dosing, postmarketing hematologic events (agranulocytosis, pancytopenia, leukopenia), CNS reactions.
  3. PEPCID (famotidine) FDA label, 2018 version (NDA 019462). — Regulatory label (Tier A). Clinical pharmacology, renal impairment exposure (at least 2-fold and 5-fold), drug interactions; no nutrient absorption mention.
  4. WHO Collaborating Centre, ATC/DDD Index: famotidine, A02BA03, DDD 40 mg. — Reference index.
  5. Lam JR, et al. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA 2013. — Nested case-control, 25,956 cases and 184,199 controls. PMID 24327038. H2 blocker OR 1.25 (1.17 to 1.34).Famotidine not named; individual H2 blockers not reported. Tier C for famotidine.
  6. Ruscin JM, Page RL, Valuck RJ. Vitamin B12 deficiency associated with histamine2-receptor antagonists and a proton-pump inhibitor. Ann Pharmacother 2002. — Case report, one patient, cimetidine then other agents. DOI 10.1345/aph.10325.Cimetidine, not famotidine. Tier C for this drug.
  7. Varughese GI, Scarpello JHB. Metformin and vitamin B12 deficiency: the role of H2 receptor antagonists and proton pump inhibitors. Age Ageing 2007;36(1):110-111. — Letter to the editor, not a study. Mentions a famotidine-treated patient and quotes a cimetidine food-bound B12 uptake study secondhand.Secondhand citation; primary cimetidine study not opened. Metformin confounds the famotidine case.
  8. Damodharan S, et al. Effect of long-term acid suppression therapy with PPIs or H2 receptor blockers on serum vitamin B12 levels in elderly population. Ir J Med Sci 2021. — Observational, 77 recruited, 60 analyzed. PMID 33085017.Ranitidine was the H2 blocker; famotidine not studied; no unexposed comparator. Tier C.
  9. Omeprazole therapy causes malabsorption of cyanocobalamin (vitamin B12). Ann Intern Med 1994;120(3). — Self-controlled trial, 10 healthy men, omeprazole 20 or 40 mg daily for 2 weeks; cyanocobalamin absorption fell from about 3% to under 1%. DOI 10.7326/0003-4819-120-3-199402010-00006.PPI study; famotidine not studied. Read for context only and not borrowed for any grade.
  10. Lam JR, et al. Proton pump inhibitor and histamine-2 receptor antagonist use and iron deficiency. Gastroenterology 2017. — Community-based case-control, 77,046 cases and 389,314 controls, 1999 to 2013. PMID 27890768. H2 blocker alone aOR 1.58 (1.46 to 1.71).Read via the Kaiser Permanente publication page. Individual H2 blockers not named; Tier C for famotidine.
  11. Streiff F. Haematological adverse effects of histamine H2-receptor antagonists (1988 review). — Narrative review; 85 cytopenia cases, 75 with cimetidine. DOI 10.1007/BF03259895. Journal name not captured in retrieval.Narrative class review without drug-level stratification. Tier C.
  12. Gao S, Song W, Lin T, Chen W, He W, Wei Q, Li Z. Prolonged use of proton pump inhibitors, but not histamine-2 receptor antagonists, is associated with lower bone mineral density in males aged over 70. Front Med 2021. — NHANES 1999 to 2006 cross-sectional, 4,232 participants aged 60 and over, 169 H2 blocker users including famotidine. DOI 10.3389/fmed.2021.725359.
  13. Anti-acid drug treatment induces changes in the gut microbiome composition of hemodialysis patients. Microorganisms 2021;9(2):286. — Cross-sectional, 193 patients (32 H2 blocker users).Famotidine not named. Tier C.
  14. Zhu J, Sun C, Li M, Hu G, Zhao X-M, Chen W-H. Compared to histamine-2 receptor antagonist, proton pump inhibitor induces stronger oral-to-gut microbial transmission and gut microbiome alterations: a randomised controlled trial. Gut 2023;72(7):1087-1097. — RCT, PPI n=23, H2 blocker n=26.Read through an aggregator summary, not the journal page (gut.bmj.com returned 429). H2 blocker agent not specified in retrieved text. Tier C.
  15. Proton pump inhibitors affect the gut microbiome (three Dutch cohorts, n=1,815). — PPI-only analysis; no H2 blocker microbiome results. Journal and year as listed at PMC (PMC4853569); not captured in retrieval.Excluded cohort: H2 blockers were not studied. Cited only to record that no H2 blocker data exist there.
  16. Biedrzycki G, Wolszczak-Biedrzycka B, Dorf J, Michalak D, Zendzian-Piotrowska M, Zalewska A, Maciejczyk M. Antioxidant and anti-glycation potential of H2 receptor antagonists: in vitro studies and a systematic literature review. Pharmaceuticals 2023;16(9):1273. — In vitro plus systematic review. DOI 10.3390/ph16091273. Two retrievals summarized the anti-glycation result differently; only the consistent statement (no more than modest effect) is used.
  17. Gholami et al. Radioprotective effects of vitamin C, cimetidine and famotidine on lipid peroxidase and hepatic glutathione levels in mouse liver. Int J Cell Biol 2025. — Animal study, 40 male NMRI mice. DOI 10.1155/ijcb/1106920.
  18. Kubiak M, Duda AM, Ganadu ML, Kozlowski H. Crystal structure of a copper(II)-famotidine complex and solution studies of the Cu2+-famotidine-histidine ternary system. J Chem Soc, Dalton Trans 1996. — In vitro coordination chemistry.
  19. Rostami et al. Effectiveness of oral famotidine in reducing the hematologic complications of radiotherapy in patients with esophageal and cardia cancers: a randomized controlled trial. Radiat Oncol 2023. — Single-blind RCT, 60 patients (30 per group), famotidine 40 mg daily and 4 hours before each session. DOI 10.1186/s13014-023-02281-w.Radioprotection trial in cancer patients; hemoglobin was a secondary comparison, so it is weak context for oxygen delivery.
  20. Essrani R, et al. Famotidine-induced hypomagnesemia leading to hypocalcemia. Oxf Med Case Rep 2020 (omz133). — Case report, one patient.
  21. Systematic review of proton pump inhibitor-associated hypomagnesemia (60 reports, 64 patients). Hindawi, 2015. — Journal path "grp" (Gastroenterology Research and Practice, inferred from the URL). Authors not captured in retrieval. Switching to H2 blockers: no recurrence in 32 (ranitidine 17, famotidine 11, cimetidine 4).
  22. Mayo Clinic. Heartburn medicines and B-12 deficiency (patient FAQ). — Tier D for evidence purposes. Used only as a circulating-claim origin.
  23. Antidote Research. Famotidine and zinc depletion (retailer page). — Tier D. Used only as a circulating-claim origin (zinc, folic acid, vitamin A, B12).