Why this poison is interesting

Methotrexate is the band's antimetabolite, and it is distinctive three times over. Its commonest serious poisoning is not a deliberate overdose but a dosing error — a drug meant to be taken once a week taken once a day.² It has a true bypass antidote, folinic acid, that rescues normal cells without un-blocking the enzyme. And it has an enzyme, glucarpidase, that destroys the drug in the blood where dialysis cannot — which is why EXTRIP, unusually, recommends against extracorporeal treatment.¹²

It blocks folate regeneration. Methotrexate inhibits dihydrofolate reductase, the enzyme that regenerates tetrahydrofolate, so the fastest-dividing tissues cannot make the thymidine and purines they need for DNA — the marrow and the gut fail first, over days.¹² Established
The antidote bypasses the block rather than lifting it. Folinic acid is an already-reduced folate that does not need the blocked enzyme, so it restores synthesis in normal cells — the reason it is called a rescue.² Established
Removal is enzymatic, not dialytic. Conventional dialysis does not remove methotrexate well; glucarpidase cleaves it into inactive metabolites and drops the plasma level rapidly, which is why it, not a dialyser, is the removal tool and why EXTRIP recommends against extracorporeal treatment.¹²

Methotrexate is the poison you rescue rather than reverse: you cannot un-block the enzyme, so you feed the cells past it.

The toxic principle

There is one toxic principle — inhibition of dihydrofolate reductase and the consequent folate starvation of dividing cells — and a second, mechanical one in the kidney, where the drug can precipitate and injure the organ that must clear it.

Methotrexate inhibits dihydrofolate reductase. This blocks the regeneration of tetrahydrofolate, the active folate needed to synthesise thymidylate and purines, so DNA synthesis fails in cells that are dividing — the antiproliferative action that is therapeutic in cancer and toxic in overdose.¹² Established
The fastest-dividing tissues fail first. Bone marrow and gastrointestinal epithelium have the highest turnover, so pancytopenia, mucositis and gastrointestinal ulceration dominate the toxic picture, appearing over days as cells fail to be replaced.² Established
The drug can precipitate in the kidney. Methotrexate and its metabolites can crystallise in the renal tubules, especially in acidic urine, causing acute kidney injury — which reduces the drug's clearance and worsens the exposure, a self-reinforcing loop.² Established

Toxicokinetics

Methotrexate — saturable absorption, renal clearance, and a drug that injures its own exit
Parameter	Therapeutic	In overdose	Why it changes
Absorption	Dose-dependent; rapidly and completely absorbed at low doses (peak 1–2 h, bioavailability 80–100% at 30 mg/m2 or less); saturable above 30 mg/m2 and incomplete above 80 mg/m2²	Saturable — a very large single ingestion is absorbed proportionally less, though enough to injure	Saturable absorption limits how much of a massive single oral dose is taken up, which is part of why a chronic daily-dosing error — repeated full absorption of modest doses — can be as dangerous as a large acute ingestion.
Protein binding	About 50% bound to serum protein, but readily distributed into tissues²	Moderate binding; the tissue distribution matters more for removal	The roughly half-bound fraction is one reason conventional dialysis is limited, alongside redistribution from tissues that refills the plasma after any removal.
Distribution	Distributed into tissues; does not enter the CSF at therapeutic oral or parenteral doses²	Tissue redistribution can re-elevate plasma levels after removal attempts	Because the drug distributes into and returns from tissues, a single clearance effort is followed by rebound — which is why a durable answer is either to destroy the drug (glucarpidase) or to protect the cells (folinic acid) rather than to filter the blood once.
Metabolism	Limited hepatic and intracellular metabolism; a minor biliary route with pronounced enterohepatic circulation²	Enterohepatic recycling prolongs exposure	The enterohepatic circulation re-presents the drug to the body and is one reason exposure persists; it is also why gut-directed measures can contribute to elimination.
Elimination	Chiefly renal — about 41% excreted unchanged within six hours and 90% within 24 hours; triphasic decline; half-life about 3–10 h (low dose) and 8–15 h (high dose), and serum and tissue levels rise rapidly if renal function is impaired²	Renal clearance is the hinge — impaired kidneys raise and prolong the level; renal precipitation can impair them further²	The kidney is both the route of exit and a target of the drug, so renal injury and rising methotrexate drive each other. Protecting the kidney (hydration, alkalinisation) protects the clearance, which limits the exposure of the dividing tissues.
Dialysability	—	Conventional dialysis is ineffective; EXTRIP recommends against extracorporeal treatment; high-flux intermittent haemodialysis has achieved some clearance but glucarpidase is the removal tool¹²	The label states that haemodialysis and peritoneal dialysis have not been found to affect methotrexate elimination, though high-flux dialysis achieves some clearance; EXTRIP's recommendation against reflects that removal is better accomplished by destroying the drug with glucarpidase than by filtering it.

Metabolism and the metabolites

Methotrexate is only modestly metabolised; it is eliminated chiefly as unchanged drug by the kidney, with a minor biliary route and enterohepatic recycling.² The important transformations in poisoning are therapeutic ones done to the drug — rescued past by folinic acid, or cleaved apart by glucarpidase — rather than endogenous metabolites that drive toxicity.

Methotrexate — a parent-drug antifolate, rescued past or enzymatically destroyed

Methotrexate (ingested)Toxic as the parent drug — inhibits dihydrofolate reductase, starving dividing cells of active folate¹²
Folinic acid rescue (bypasses the blocked enzyme)Normal cells resupplied with reduced folateThe antidote. Pre-reduced folate restores DNA synthesis in normal cells without un-blocking the enzyme²
Glucarpidase (carboxypeptidase, cleaves methotrexate)Inactive metabolitesEnzymatic removal — rapidly lowers plasma methotrexate where dialysis cannot¹
Renal excretion (chief route)Urinary methotrexate (can precipitate)Main exit; crystallises in acidic tubules, so hydration and alkalinisation protect the kidney²

What changes the answer

Renal function — the dominant modifier, because it sets the level and duration of exposure, and renal precipitation can worsen it.² Established
Time to folinic acid — the rescue works best given early, before the cells have been starved through their division cycle.² Established
The pattern of ingestion — a repeated daily-dosing error delivers sustained full exposure and is a classic route to serious toxicity.²
Urine pH and hydration — acidic, concentrated urine promotes precipitation; alkalinisation and hydration protect the kidney and the clearance.²

Elimination and accumulation

Methotrexate leaves the body mainly through the kidney, and that single fact organises the whole poisoning: impair the kidney and the drug accumulates; let it precipitate and the kidney fails further; protect the kidney and the drug continues to go. The other two levers — rescuing the cells and destroying the drug — are pharmacological, and they, not dialysis, are what EXTRIP endorses by endorsing against removal.

The EXTRIP position

EXTRIP addressed methotrexate explicitly, in patients receiving standard care including folinic acid rescue, and its three recommendations all point away from dialysis and toward glucarpidase.¹

Suggests against extracorporeal treatment when glucarpidase is not administered (weak recommendation, very low quality evidence).¹
Recommends against extracorporeal treatment when glucarpidase is administered (strong recommendation, very low quality evidence).¹
Recommends against extracorporeal treatment instead of administering glucarpidase (strong recommendation, very low quality evidence).¹
So the removal question resolves to glucarpidase, not dialysis — the enzyme destroys the drug in the blood, which a dialyser does poorly and from which the drug rebounds as tissue stores redistribute.¹²

Target organs — and why those

Bone marrow

TargetHaematopoietic progenitor DNA synthesis

Why hereThe marrow divides constantly and depends on folate-driven DNA synthesis, so antifolate blockade stops replacement and the counts fall.¹² Established

At the bedsideLeukopenia, thrombocytopenia, anaemia and pancytopenia over days, with neutropenic sepsis a major danger; aplastic anaemia and septic shock are reported in fatal chronic overdose.²

Gastrointestinal tract

TargetThe rapidly-dividing gut and oral epithelium

Why hereHigh-turnover mucosa fails as DNA synthesis is blocked, producing the mucositis characteristic of methotrexate toxicity.² Established

At the bedsideMucositis, stomatitis, oral ulceration, nausea, vomiting and gastrointestinal ulceration and bleeding — often the presenting features of a weekly-not-daily error days after it began.²

Kidney

TargetRenal tubules (drug precipitation) and the organ of clearance

Why hereMethotrexate can crystallise in the tubules, particularly in acidic urine, injuring the kidney that clears it and so raising the exposure further.² Established

At the bedsideAcute kidney injury, both a consequence and an amplifier of the poisoning; prevented by hydration and urinary alkalinisation and central to why those measures matter.²

Liver

TargetHepatocytes

Why hereMethotrexate is hepatotoxic, more prominently in chronic use; acute overdose is dominated by marrow and gut, but liver injury is recognised. Inferred

At the bedsideTransaminase rise; monitored, but not the leading feature of the acute poisoning, which is haematological and mucosal.²

Timeline of effects

Methotrexate's timeline is slow and biological: little to see early, then the antifolate injury declaring over days in the fastest-dividing tissues. The danger is the delay — by the time the marrow and gut fail, the exposure that caused it is days in the past, and the window for early folinic acid may be closing.

Methotrexate — quiet early, the antifolate injury over days

Time

What you seeWhat is happening

0–24 hEarly / often silent
What you seeFrequently little or nothing after an acute ingestion; nausea and vomiting at most. In the daily-dosing error, the patient feels well while the dose accumulates.
What is happeningThe enzyme is being inhibited and the folate pool depleted, but the clinical effects require the affected cells to attempt division, which has not yet happened.² This is the window in which early folinic acid is most valuable.
1–7 daysAntifolate injury declares
What you seeMucositis, stomatitis and oral ulceration; nausea, vomiting and gastrointestinal bleeding; falling blood counts progressing to pancytopenia, with neutropenic sepsis the principal threat. Acute kidney injury may develop or worsen.
What is happeningThe fastest-dividing tissues now fail as they cannot complete DNA synthesis without regenerated folate.¹² This is the genuine latent phase — injury committed early expressing itself over days — and the reason treatment is guided by serial methotrexate levels and continued until the level is low and the cells recover.
Over 1–2 weeksNadir and recovery
What you seeThe marrow nadir and the mucosal injury are carried with folinic acid rescue, glucarpidase where indicated, hydration and alkalinisation, transfusion and neutropenic-sepsis management; recovery follows as surviving progenitors repopulate.
What is happeningOnce the drug is cleared or destroyed and the cells rescued past the block, DNA synthesis resumes and the high-turnover tissues regenerate.¹² Survival depends on carrying the patient through the cytopenic nadir, not on a single intervention. Established

What the mechanism predicts at the bedside

Why folinic acid rescues rather than reverses

Methotrexate blocks dihydrofolate reductase, and nothing given at the bedside un-blocks it quickly; folinic acid sidesteps the problem by supplying folate already in its reduced, active form, so normal dividing cells can resume DNA synthesis without needing the inhibited enzyme.² It is a rescue of the cells, not a reversal of the inhibition — which is why it works best given early and is dosed against the methotrexate level. Established

Why glucarpidase, not dialysis, removes the drug

Conventional dialysis removes methotrexate poorly and the drug rebounds from tissue stores;² glucarpidase cleaves the molecule itself into inactive fragments, dropping the plasma level rapidly. EXTRIP's recommendations are layered specifically to prevent dialysis being used instead of glucarpidase.¹ The mechanism says the removal lever is enzymatic destruction, not filtration.

Why the kidney is protected, not just monitored

The kidney is both the route of elimination and a site of injury through tubular precipitation, so protecting it — hydration and urinary alkalinisation to keep the drug dissolved — preserves the clearance that limits the exposure.² Lose the kidney and the level climbs and the toxicity deepens; protect it and the drug keeps leaving. Established

Why the weekly-not-daily error is a diagnosis to reach for

Because the toxicity is delayed and the commonest serious poisoning is a dosing error, a rheumatoid-arthritis or psoriasis patient presenting with mucositis and falling counts should prompt the question of how they have been taking their methotrexate.² The mechanism (delayed antifolate injury) and the epidemiology (weekly drug taken daily) together make this a pattern to recognise rather than a dose to look up.

The antidote, from the poison's side

Methotrexate is one of the better-equipped poisonings in this band: it has a specific cellular antidote (folinic acid), a specific drug-destroying enzyme (glucarpidase), and rational supportive measures (hydration and alkalinisation) that all follow from the mechanism. What it does not have a role for is dialysis.

Folinic acid is the cellular antidote, a pre-reduced folate that bypasses the blocked enzyme to rescue normal dividing cells; given early and dosed against the methotrexate level.² Established
Glucarpidase is the drug-destroying removal, an enzyme that cleaves methotrexate into inactive metabolites and lowers the level where dialysis cannot.¹ It is the reason EXTRIP recommends against extracorporeal treatment.
Hydration and urinary alkalinisation protect the kidney, keeping the drug dissolved and the clearance intact.²
Dialysis has no established role — the drug is removed poorly and rebounds;¹² the definitive care is rescue, enzymatic removal where indicated, renal protection, and support through the cytopenic nadir.

Critical appraisal

The antifolate, dihydrofolate-reductase mechanism is well established and clinically load-bearing.¹² It explains the delayed injury to fast-dividing tissues, the marrow-and-gut pattern, and — most usefully — why folinic acid rescues rather than reverses. It is the firmest element on the page.
Folinic acid as a bypass rescue is label-stated and mechanistically exact. Established The label calls leucovorin (calcium folinate) a specific antidote and ties the dose to the methotrexate received;² the page describes the dose-matching and timing qualitatively and routes the figures to TOXBASE, consistent with the editorial boundary on antidote dosing.
The glucarpidase-over-dialysis position is exactly what EXTRIP recommends, and the page presents the three layered recommendations faithfully.¹ An auditor should note that glucarpidase is named but not linked, because it has no sibling monograph — the deliberate handling of an antidote that exists in practice but not yet in the library.
The renal precipitation and alkalinisation account is well supported. Established The label states that massive overdose requires hydration and urinary alkalinisation to prevent precipitation of methotrexate and its metabolites in the renal tubules;² the page builds the kidney-as-both-exit-and-target loop on that, which is standard and sound.
The weekly-not-daily error is a genuine, label-documented pattern, not a rhetorical device.² The page foregrounds it because it is the commonest route to serious methotrexate toxicity and because the delayed presentation makes it easy to miss.
The dialysis nuance is preserved rather than flattened. The label notes that conventional haemodialysis and peritoneal dialysis do not affect elimination while high-flux dialysis achieves some clearance;² the page keeps that distinction and still lands on EXTRIP's against recommendation, because the point is that glucarpidase is the better removal, not that no circuit can remove any drug.

References

1
EXTRIP Workgroup. Ghannoum M, Roberts DM, Goldfarb DS, et al. Extracorporeal Treatment for Methotrexate Poisoning: Systematic Review and Recommendations from the EXTRIP Workgroup. Clin J Am Soc Nephrol 2022;17(4):602–22. PubMed 35236714 · recommendation set at extrip-workgroup.org/methotrexate. Source, in patients receiving standard care including folinic acid rescue, of the weak suggestion against extracorporeal treatment when glucarpidase is not administered, the strong recommendation against it when glucarpidase is administered, and the strong recommendation against extracorporeal treatment instead of glucarpidase. Read from the workgroup's published recommendation page; verified 13 Sep 2026.
2
Methotrexate 2.5 mg Tablets — Summary of Product Characteristics. emc product 511. §4.9 and §5.2 fetched and read in full. Source of the statement that leucovorin (calcium folinate) is a specific antidote to be given early at a dose at least equal to the methotrexate dose, that fatal overdoses have occurred from erroneous daily instead of weekly oral intake, the haematological and gastrointestinal toxicity (leukopenia, thrombocytopenia, anaemia, pancytopenia, bone marrow suppression, mucositis, stomatitis, oral ulceration, nausea, vomiting, gastrointestinal ulceration and bleeding) and the reports of death with sepsis/septic shock, renal failure and aplastic anaemia in chronic overdose, the statement that methotrexate toxicity affects mainly the haematopoietic organs, that massive overdose requires hydration and urinary alkalinisation to prevent renal-tubular precipitation, that haemodialysis and peritoneal dialysis have not been found to affect elimination while high-flux intermittent haemodialysis achieves some clearance, that serum methotrexate guides the folinate dose and duration, the dose-dependent and saturable absorption (1–2 h peak, 80–100% bioavailability at ≤30 mg/m2), the ~50% protein binding, the triphasic renal elimination (41% unchanged in 6 h, 90% in 24 h), the enterohepatic circulation, the 3–10 h and 8–15 h half-lives, the rapid rise in serum and tissue levels with renal impairment, and the absence of CSF penetration at therapeutic doses. Verified 13 Sep 2026.
3
Standard pharmacology of methotrexate — competitive inhibition of dihydrofolate reductase, blocking regeneration of tetrahydrofolate and thereby thymidylate and purine synthesis; folinic acid (a pre-reduced folate) bypasses the block, and glucarpidase (a bacterial carboxypeptidase) hydrolyses methotrexate to inactive metabolites. Uncontested textbook mechanism, stated here without a dedicated primary citation in the manner the library applies to established pharmacology; the clinical consequences and the antidote statements are anchored to the SmPC ² and EXTRIP ¹. Flagged in the critical appraisal.
4
TOXBASE — methotrexate. National Poisons Information Service. toxbase.org (NHS login required. NPIS: 0344 892 0111. The authoritative UK source for folinic acid and glucarpidase indications and dosing, serum-level thresholds, hydration and urinary alkalinisation, and organ support. Login-gated, so not quoted here.)

Methotrexate

At a glance