Folinic acid

Why this drug is interesting

Methotrexate inhibits dihydrofolate reductase, so the cell cannot reduce folate to tetrahydrofolate and cannot make thymidine or purines. Folate itself is therefore useless as an antidote — it needs the blocked enzyme to become active. Folinic acid is already reduced. It enters the folate pool downstream of the blockade, and the cell resumes making DNA.

That single distinction — folic acid will not work, folinic acid will — is the most important thing on this page and the commonest drug-name error in this area of practice.

Pharmacology

Mechanism

Folinic acid is 5-formyl-tetrahydrofolate. It is converted to the other reduced folate cofactors without passing through dihydrofolate reductase, restoring thymidylate and purine synthesis in cells whose folate cycle methotrexate has arrested.

Crucially, it does not remove methotrexate. It competes with it. High intracellular methotrexate concentrations can outcompete a standard folinic acid dose, which is exactly why the rescue dose escalates with the measured methotrexate level rather than staying fixed.

Saturable absorption

"Dosages above 25–50 mg should be given parenterally due to saturable enteral absorption of calcium folinate."¹ Above that threshold, an oral dose does not deliver a proportionally larger amount. Rescue must also be parenteral in malabsorption syndromes or other gastrointestinal disorders where enteral absorption is not assured.

Dosing — methotrexate rescue

When rescue is required

• Necessary when methotrexate is given at doses exceeding 500 mg/m².¹
• To be considered at doses of 100–500 mg/m².¹

The standard regimen

"As a rule, the first dose of calcium folinate is 15 mg (6–12 mg/m²) to be given 12–24 hours (24 hours at the latest) after the beginning of the methotrexate infusion. The same dose is given every 6 hours throughout a period of 72 hours."¹ After several parenteral doses, treatment can switch to the oral form — subject to the 25–50 mg absorption ceiling.

Escalation by the 48-hour level

Measure the residual methotrexate level 48 hours after the start of the methotrexate infusion. If it is above 0.5 µmol/L, escalate:¹

Continue for 48 hours, or until the methotrexate level falls below 0.05 µmol/L.¹

What runs alongside it

"In addition to calcium folinate administration, measures to ensure the rapid excretion of methotrexate — maintenance of high urine output and alkalinisation of urine — are integral parts of the Calcium Folinate Rescue treatment. Renal function should be monitored by measuring serum creatinine levels daily."¹

Glucarpidase — the other methotrexate antidote

Glucarpidase (Voraxaze) is a recombinant bacterial carboxypeptidase that cleaves methotrexate into inactive DAMPA and glutamate. It is licensed "to reduce toxic plasma methotrexate concentration in adults and children (aged 28 days and older) with delayed methotrexate elimination".² Where folinic acid rescues the cell, glucarpidase destroys the drug.

Dose

A single dose of 50 units/kg by intravenous bolus over 5 minutes²

Timing

Administration "should optimally occur within 60 hours from the start of the high-dose methotrexate infusion, because life-threatening toxicities may not be preventable beyond this time point" — though clinical data show continued effectiveness beyond that window²

Threshold examples

For methotrexate 8–12 g/m² over ≤6 h: ≥50 µmol/L at 24 h. For 1–8 g/m² over 24 h: ≥30 µmol/L at 36 h, ≥10 at 42 h. For the ≤1, 1–8 and 8–12 g/m² regimens tabulated: ≥5 µmol/L at 48 h; a separate short-infusion table gives ≥6 µmol/L at 48 h for 5 g/m²²

Renal impairment

No dose adjustment²

Paediatric

No dose adjustment; no data below 28 days²

Glucarpidase breaks the methotrexate assay

The label's practical instructions follow directly:²

• HPLC is the recommended method for methotrexate measurement after glucarpidase.
• In the absence of HPLC, base the folinic acid dose for the 48 hours after glucarpidase on the methotrexate concentration from a sample taken before it was given.
• Within 48 hours, immunoassay results cannot reliably be used to monitor for rebound; seek confirmatory HPLC.
• Beyond 48 hours, immunoassay is reliable in most patients and can again be used to adjust folinic acid and monitor rebound.

The non-oncology indications

The licensed indications are "to diminish the toxicity and counteract the action of folic acid antagonists such as methotrexate in cytotoxic therapy and overdose, in adults and children", and use in combination with 5-fluorouracil.¹ The word overdose matters: deliberate or accidental methotrexate overdose — including the classic weekly-dose-taken-daily error — is a labelled indication.

Methotrexate dosing errors

The most likely ED presentation. A patient prescribed weekly methotrexate who has taken it daily may present days later with mucositis, pancytopenia and sepsis. Folinic acid rescue, hydration and haematology involvement, urgently

Trimethoprim and co-trimoxazole toxicity

Both are dihydrofolate reductase inhibitors; folinic acid is used in high-dose or prolonged therapy and in overdose with marrow suppression

Pyrimethamine

Another DHFR inhibitor; folinic acid is co-prescribed routinely in toxoplasmosis treatment

Methanol poisoning

Folinic acid (or folic acid) accelerates the conversion of formate to carbon dioxide and water, and is given adjunctively alongside fomepizole and dialysis

Safety — the one thing that must not go wrong

Contraindication

Pernicious anaemia or other vitamin B₁₂ deficiency anaemia.¹ Folinate can produce a haematological response while the underlying B₁₂ deficiency and its neurological consequences progress — the label separately warns that folinate may mask these anaemias.

Rate and interactions

• Maximum intravenous injection rate 160 mg/minute, because of the calcium content of the solution.¹
• Antiepileptics — phenobarbital, phenytoin, primidone and succinimides: folinate may reduce their plasma concentrations and increase seizure frequency.¹
• Folinic acid enhances the toxicity of 5-fluorouracil, which is the basis of its licensed combination use — relevant if a patient on that combination presents unwell.¹

Practical use in the ED

1. Ask how the methotrexate is taken. "Once a week" is the answer you are checking against. A daily-dosing error is a medical emergency even if the patient looks well.
2. Take bloods early — full blood count, renal function, liver function, and a methotrexate level where the laboratory can provide one. The level guides the dose.
3. Start folinic acid without waiting where the history is convincing. It is a low-harm drug and the window is time-critical.
4. Give it parenterally if the dose exceeds 25–50 mg, or if absorption is not assured — which in a patient with mucositis and vomiting, it is not.
5. Hydrate and alkalinise, and check creatinine daily.
6. Do not give activated charcoal reflexively. The 2026 CTRC guideline recommends charcoal for methotrexate above an ingested dose of 10 mg/kg,³ so it may be indicated in acute oral overdose — but it has no role in the patient presenting days into a dosing error.
7. Involve haematology or oncology, and NPIS. The decision to escalate the rescue dose, or to seek glucarpidase, is not one to make alone — and glucarpidase supply requires notice.

Critical appraisal

Folinic acid rescue is one of the few antidotes whose regimen is genuinely established, because it has been given to hundreds of thousands of patients within structured oncology protocols with measured drug levels. The mechanism is exact, the dose–level relationship is explicit, and the endpoint is a number. Compared with almost everything else in this batch, it is on solid ground.

The weaknesses are at the edges. The label defers dosing to the individual methotrexate protocol, which is correct for oncology and unhelpful in an emergency department seeing a dosing error with no protocol at all. The illustrative regimen — 15 mg six-hourly, escalating by 48-hour level — is what is left to work from, and it was designed for planned high-dose therapy rather than for an unplanned ingestion of unknown timing.

Glucarpidase, by contrast, has the profile of a modern biologic antidote: a clean mechanism, a single fixed weight-based dose, no renal adjustment — and a laboratory interference that renders the monitoring test unreliable for two days afterwards. It is also expensive and not stocked everywhere, so the practical question is again where the nearest vials are.

References

1. 1
   Calcium Folinate 10 mg/ml Solution for Injection or Infusion — Summary of Product Characteristics, Sandoz Limited. electronic Medicines Compendium, product 9649. Sections 4.1, 4.2. Source of the licensed indications including overdose, the 15 mg (6–12 mg/m²) six-hourly regimen, the 12–24 hour start and 24-hour late limit, the 500 mg/m² and 100–500 mg/m² thresholds, the 48-hour escalation table, the 25–50 mg parenteral threshold and the hydration/alkalinisation requirement. Verified 31 Aug 2026. Five calcium folinate or folinic acid products were listed on the emc at that date, Sodiofolin (product 1296) is disodium folinate, whose doses are expressed in folinic acid equivalents on the same milligram scale as calcium folinate — its section 4.2 gives “500 mg/m² folinic acid (= 546.5 mg/m² disodium folinate)”. The levofolinic acid products are the l-isomer and are dosed at roughly half the calcium folinate figure; those milligram doses are not interchangeable. Verified 4 Sep 2026.
2. 2
   Voraxaze 1,000 units powder for solution for injection (glucarpidase) — Summary of Product Characteristics, SERB. electronic Medicines Compendium, product 102214. Sections 4.2, 4.4. Source of the 50 units/kg single dose over 5 minutes, the 48–60 hour window, the methotrexate threshold tables, the two-hour folinic acid separation rule and the DAMPA immunoassay interference including the 8.6 hour half-life and 48-hour guidance. Verified 31 Aug 2026.
3. 3
   Hoegberg LCG, Gosselin S, Buckley NA, Wood DM, et al. Recommendations from the Clinical Toxicology Recommendations Collaborative on the administration of activated charcoal in acute oral overdose. Clinical Toxicology 2026;64(6):419–475. PubMed 41906697. Table 4: methotrexate single-dose threshold 10 mg/kg, GRADE D; additional-dose charcoal weakly recommended against. Full text read 4 Sep 2026.