Probenecid

Limits on what this shows

• Null findings: Rat liver basolateral membrane vesicles: no effect on riboflavin uptake by probenecid (PMID 7794963); human colonic NCM460 cells: riboflavin uptake inhibited by amiloride but not by probenecid (PMID 10666022) [7]. Probenecid therefore does not block riboflavin transport in every tissue tested, including the gut-derived cell line, which does not support an intestinal uptake mechanism in that model.
• At-risk subgroups: None identified. Renal impairment would alter both urate handling and probenecid exposure per the label [1].

Regulatory status

No riboflavin statement in the label text read [1].

Population evidence

Human: two studies were found by title only, "Effect of probenecid on riboflavin absorption and excretion in man" (J Pharm Sci 1967, PMID 6049701) and "Effect of probenecid on renal clearance of riboflavin in man" (Jusko et al., J Pharm Sci 1970, PMID 5440673). The 1970 PubMed record carries MeSH terms (probenecid pharmacology, riboflavin in blood and urine, kidney tubules, protein binding) but no abstract; the 1967 record was not opened. Their results, sample sizes and direction are unknown to this review [7][8]. Preclinical, listing-level abstracts: MDCK cells, 1 mM probenecid reduced riboflavin transport to 54% of control (PMID 2402010); riboflavin accumulation by isolated choroid plexus was inhibited by probenecid (PMID 489597); probenecid in the injectate decreased clearance of [14C]riboflavin from cerebrospinal fluid (PMID 7452254); a Xenopus oocyte study found dose-dependent suppression judged nonspecific (PMID 7880856) [7]. A 2025 zebrafish model of riboflavin transporter deficiency found riboflavin alone improved outcomes and added probenecid gave modest extra benefit to hearing, which is a disease-model result [7].

Mechanism

Named, partly: competition at probenecid-sensitive anion transport in renal and other epithelia. The tissues differ: transport was inhibited in kidney-derived MDCK cells and choroid plexus.

Direction

Retention or altered excretion at most (transport competition); not depletion. Unproven in people from any text read.

Counter-evidence held on record

Counter-evidence and confounding Cell concentrations are millimolar and not reconciled to human plasma levels. Human studies were not readable. Dietary riboflavin intake and gout comorbidity were not assessed in any source read. Framing caution Not a riboflavin deficiency claim, and not a reason to take riboflavin. If the human data later show reduced urinary excretion, the signal would be retention, not loss.

Limits on what this shows

• Null findings: The rat kidney study found no effect on 5-methyltetrahydrofolate reabsorption [9]. No human serum or red-cell folate study with probenecid was found in the listing returned.
• At-risk subgroups: Patients receiving methotrexate (label interaction). Renal impairment would further reduce methotrexate clearance; not quantified in any source read.

Regulatory status

Label Drug Interactions: "Probenecid increases plasma concentrations of methotrexate in both animals and humans." One summarizer pass also reported a recommendation to reduce the methotrexate dose; that sentence was not repeated in the verbatim-quote pass and is not relied on [1].

Population evidence

Methotrexate (drug-to-drug), listing-level: a 1985 J Rheumatol Suppl review states salicylates and probenecid may decrease plasma protein binding and renal tubular secretion of methotrexate; a 1976 rat study found increased methotrexate toxicity with concurrent probenecid (PMID 942450); a 2023 critical review of methotrexate interactions lists probenecid among drugs raising methotrexate levels through transporter inhibition with case reports predominating (PMID 36946211); primate CSF exposure of methotrexate rose 3.2-fold with intralumbar probenecid (PMID 11592346) [12]. Folate itself, all Tier B: in cultured rat proximal tubule cells 10 mM probenecid reduced 5-methyltetrahydrofolate transport by 50% (PMID 9744553); in the isolated perfused rat kidney probenecid did not affect 5-methyltetrahydrofolate reabsorption but inhibited urinary excretion of its metabolites (PMID 8558307; the abstract-level read did not give the results, which come from the PubMed listing) [9][10]; probenecid reduced folic acid uptake in Plasmodium falciparum and raised antifolate susceptibility 3 to 7-fold at 50 microM (PMID 12821454) [11]; probenecid-sensitive 5-methyltetrahydrofolate carriers were described in monkey MA104 cells and human placental JAR cells (PMIDs 1849150, 8031869) [9].

Mechanism

Named: competition at renal tubular organic anion secretion (methotrexate) and probenecid-sensitive folate carriers (cell studies). The tubular studies suggest probenecid slows urinary folate metabolite excretion, which points to retention, not loss.

Direction

Hard Rule 4: the probenecid to methotrexate effect is drug-drug (exposure of an antifolate drug rises), not a nutrient depletion. For folate itself the preclinical direction is retention or unchanged, and no human value exists.

Counter-evidence held on record

Counter-evidence and confounding Folic acid supplementation is given with methotrexate for its toxicity and is unrelated to probenecid. Rat and cell concentrations (10 mM) exceed clinical exposure. Most methotrexate evidence is listing-level and from case reports and reviews. Framing caution This is a drug interaction prompt for prescribers, not a folate deficiency claim. No folate dose is implied.

Limits on what this shows

• Null findings: None found; the absence of numbers reflects what the listing text showed, not a measured low risk.
• At-risk subgroups: Label: known uric acid stone formers (not recommended); acid urine and low fluid intake.

Regulatory status

Label: increases "the urinary excretion of uric acid"; Contraindications: "not recommended in persons with known blood dyscrasias or uric acid kidney stones"; adverse reactions "uric acid stones with or without hematuria, renal colic, costovertebral pain, urinary frequency"; "As uric acid tends to crystallize out of an acid urine, a liberal fluid intake is recommended", with urine alkalinization named as a preventive measure [1]. Drugs.com gives no stone frequency [37].

Population evidence

No incidence or odds ratio for stones with probenecid was found. A PubMed listing for probenecid with uricosuric urolithiasis risk returned four reviews (PMIDs 27591828, 30915866, 21139282, 12629718) that mention urolithiasis and probenecid without patient numbers or stone outcomes in the listing text [20]. An earlier listing combining stone and hypercalciuria terms returned only general hypercalciuria reviews with no probenecid data [19]. Steele and Boner 1973 (healthy men) studied how probenecid and chlorothiazide lower serum urate; the abstract does not describe stone outcomes [23].

Mechanism

Named: urate reabsorption blockade raises urinary urate load; acid urine favors crystallization (label).

Direction

Intended pharmacologic effect and outcome (excess urinary urate), not a nutrient deficiency (Hard Rule 2).

Counter-evidence held on record

Counter-evidence and confounding Hyperuricemia and gout themselves raise uric acid stone risk (confounding by indication, Hard Rule 6); the label advice on fluid and alkalinization addresses that risk. Calcium stones were not assessed. Framing caution Prescriber and hydration context only. No supplement is implied, and any urine-alkalinizing agent belongs under prescriber direction.

Limits on what this shows

• Null findings: No population rate or controlled study found.
• At-risk subgroups: None defined in the sources read.

Regulatory status

Label Adverse Reactions, genitourinary: "nephrotic syndrome, uric acid stones with or without hematuria, renal colic, costovertebral pain, urinary frequency" [1]. Drugs.com: "nephrotic syndrome, with or without renal failure", frequency not reported [37].

Population evidence

Case reports and one note, listing-level titles: "Nephrotic syndrome caused by probenecid" (N Engl J Med 1961, PMID 13699173), "Nephrotic syndrome caused by probenecid" (JAMA 1967, PMID 6071127), "Probenecid-induced nephrotic syndrome" (Hertz et al., Arch Pathol 1972, PMID 5051645; case report with kidney biopsy microscopy per OpenAlex metadata; no abstract text available), "Probenecid, nephrotic syndrome, and renal failure" (Ann Rheum Dis 1968, PMID 5655316) [17][18]. The results of these reports were not read. A 2010 study of probenecid with mizoribine in nephrotic syndrome patients concerns drug pharmacokinetics (PMID 20979934) [17].

Mechanism

Not established in any source read. The case report with immunofluorescence suggests an immune-mediated pattern, but its findings were not read.

Direction

Not a nutrient direction. A nephrotic state would cause urinary loss of proteins and protein-bound substances, but no source measured vitamin D, zinc, copper or any nutrient in a probenecid-associated case, so no nutrient entry is made (Hard Rule 2).

Counter-evidence held on record

Counter-evidence and confounding Very old single-patient reports, results unread. Causality in individual cases was not evaluated here. Gout with CKD is itself associated with proteinuria. Framing caution Prompt for urinalysis or edema evaluation by the prescriber, not a nutrient claim.

Limits on what this shows

• Null findings: The label states no effect in euparathyroid individuals. A rat study found probenecid did not alter diphosphonate renal clearance (PMID 240057), a different molecule [21].
• At-risk subgroups: Hypoparathyroid patients (label). Renal impairment not assessed in any source read.

Regulatory status

Label: "The tubular reabsorption of phosphorus is inhibited in hypoparathyroid but not in euparathyroid individuals" [1].

Population evidence

Listing-level titles of human studies: "The effect of probenecid (benemid) on phosphate excretion and other metabolic processes" (Spurr et al., Am J Med Sci 1954, PMID 13197374; no abstract available) [22], and "Influence of benemid on urinary excretion of phosphate in hypoparathyroidism" (Metabolism 1954, PMID 13203058; not opened) [21]. A GeneReviews chapter on hyperphosphatemic familial tumoral calcinosis lists probenecid among agents used to increase renal phosphate excretion (PMID 29389098, listing text) [21]. In fumaric acid ester Fanconi syndrome (10 patients) two had a favorable biochemical response to probenecid; the fractional excretion of phosphate was 24.2% (PMID 35261762, listing text) [21]. A 1973 study in healthy men found chlorothiazide and probenecid produced comparable phosphaturic responses by one listing summary (PMID 4703224); the opened abstract speaks only of sodium and phosphorus excretion under chlorothiazide and does not confirm that comparison [23].

Mechanism

Named, label level: inhibition of tubular phosphorus reabsorption in hypoparathyroid individuals; no named transporter.

Direction

Increased urinary phosphate excretion in a defined subgroup (renal loss), with no measured serum phosphate fall in any source read. A tubular drug effect, not a dietary deficiency.

Counter-evidence held on record

Counter-evidence and confounding The human studies are from 1954 and their results were not read. Hypoparathyroidism is rare in gout cohorts, so relevance to typical users is limited. Calcium handling was not found in any opened source (see Calcium, ungraded). Framing caution No phosphate supplementation or restriction is implied. Phosphate monitoring would apply only to hypoparathyroid or renally impaired users, at prescriber discretion.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C source opened and no circulating zinc claim found. A disclosed gap, not a finding of safety. A nephrotic or hemolytic state could change zinc handling but no source linked either to zinc for probenecid.

What was searched

Label keyword check (zinc absent) [1]; unfocused metal listing [35]; PubMed "probenecid zinc" and "probenecid trace elements" (both 429, not read); Drugs.com pages.

Channel outcomes

The two focused zinc queries were rate-limited and were not read. Tooling-limited; re-check when PubMed access allows. Repair pass 2026-10-07: WebSearch returned a refusal stating the shared search budget was used up, so no web search ran. WebFetch requests to a PubMed listing, the OpenAlex works search, the Crossref bibliographic search and the Europe PMC REST search were each refused with HTTP 429 (the proxy asked for a wait of about 60 seconds and said not to refetch the page); the PubMed listing request was repeated after waits of about 2, 4 and 5 minutes and was refused each time, so no new record was read. The PubMed "probenecid zinc" listing was one of the requests repeated and refused. Planned but unrun: "probenecid zinc urinary excretion", "probenecid serum zinc". Classification: tooling-limited gap.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C calcium evidence and no circulating calcium claim for probenecid found. Calcium stones were not assessed; the stone entry covers uric acid stones only. A disclosed gap, not a finding of safety.

What was searched

Label keyword check (calcium absent) [1]; PubMed listing "probenecid uric acid stones OR nephrolithiasis OR hypercalciuria" (10 general hypercalciuria and stone reviews with no probenecid data) [19]; "probenecid phosphate OR calcium excretion" (429, not read); "probenecid phosphate excretion" (read, phosphate entry; no calcium result) [21]; Drugs.com pages.

Channel outcomes

Calcium-specific probenecid queries returned unrelated reviews or were rate-limited. Tooling-limited, probably a thin literature. Repair pass 2026-10-07: WebSearch returned a refusal stating the shared search budget was used up, so no web search ran. WebFetch requests to a PubMed listing, the OpenAlex works search, the Crossref bibliographic search and the Europe PMC REST search were each refused with HTTP 429 (the proxy asked for a wait of about 60 seconds and said not to refetch the page); the PubMed listing request was repeated after waits of about 2, 4 and 5 minutes and was refused each time, so no new record was read. Planned but unrun: PubMed "probenecid urinary calcium", "probenecid hypercalciuria", "probenecid serum calcium". Classification: tooling-limited gap. The only calcium-adjacent human data seen remain the 1954 phosphate titles used in the Phosphate entry, whose results were not read.

Limits on what this shows

• Why no grade: Dual condition met: no Tier B or C evidence located and no circulating claim found. A disclosed gap, not a finding of safety. Re-check each vitamin individually when access allows.

What was searched

Label keyword check (vitamin absent) [1]; PubMed listing "probenecid vitamin" (10 records, none examined probenecid effects on vitamin excretion, plasma levels or transport) [34]; "probenecid ascorbic acid" (429, not read); Drugs.com pages [36].

Channel outcomes

Only a generic vitamin query was completed; vitamin C, B6, B12, thiamine and pantothenate were not individually searched except for the rate-limited ascorbic acid query. Tooling-limited. Repair pass 2026-10-07: WebSearch returned a refusal stating the shared search budget was used up, so no web search ran. WebFetch requests to a PubMed listing, the OpenAlex works search, the Crossref bibliographic search and the Europe PMC REST search were each refused with HTTP 429 (the proxy asked for a wait of about 60 seconds and said not to refetch the page); the PubMed listing request was repeated after waits of about 2, 4 and 5 minutes and was refused each time, so no new record was read. The PubMed "probenecid ascorbic acid" listing was among the requests repeated and refused. Planned but unrun: individual PubMed queries for probenecid with vitamin C (ascorbic acid), pyridoxine or vitamin B6, vitamin B12 or cobalamin, thiamine, and pantothenic acid or pantothenate (pantothenate is an organic anion, so transport competition at renal organic anion transporters is a plausible lead to test). Classification: tooling-limited gap, with the unrun pantothenate and vitamin C queries the highest-yield leads.