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# Folic acid
## Overview
Folic acid (vitamin B9) is a water-soluble B vitamin essential for DNA synthesis, repair, and erythropoiesis. It is inactive in its supplemental form and requires conversion to tetrahydrofolate by dihydrofolate reductase.
## Primary Indications
* Treatment and prophylaxis of folate deficiency anemia.
* Prevention of neural tube defects (NTDs) during pregnancy.
* Reduction of hematologic toxicity associated with methotrexate therapy.
* Adjunct in hyperhomocysteinemia.
## Adult Dosing
* **Folate Deficiency:** 1 mg daily (range 1–5 mg/day) until hematologic recovery.
* **Prophylaxis (Pregnancy):** 400 mcg to 800 mcg daily. High-risk patients (history of NTD-affected pregnancy) may require 4–5 mg daily (per clinical protocol).
* **Methotrexate-related Toxicity:** 1 mg daily or 5 mg weekly (not on day of methotrexate administration).
## Pediatric Dosing
* **Folate Deficiency (Infants):** 0.1 mg daily.
* **Folate Deficiency (Children 1–12 years):** 0.1 mg to 0.4 mg daily.
* **Maintenance:** 0.1 mg daily.
* *Note: Dosing should be individualized based on severity and underlying etiology; consult institutional pediatric protocols.*
## Dose Adjustments
* **Renal/Hepatic Impairment:** No standard adjustments required.
* **Malabsorption Syndromes:** May require significantly higher doses (up to 15 mg/day) if caused by conditions like tropical sprue.
## Contraindications
* Known hypersensitivity to folic acid.
* **Critical Warning:** Do not use as monotherapy for pernicious anemia (B12 deficiency) or other megaloblastic anemias where B12 is deficient. Folic acid may mask the hematologic symptoms of B12 deficiency while allowing irreversible neurological damage to progress.
## Adverse Effects
Generally well-tolerated.
* **Rare:** Allergic reactions (rash, pruritus, dyspnea), gastrointestinal distress (nausea, abdominal distension), and bitter taste.
## Key Drug Interactions
* **Anticonvulsants (Phenytoin, Carbamazepine, Valproate):** Folic acid may decrease serum concentrations of these agents, potentially reducing seizure control.
* **Methotrexate:** High doses of folic acid may interfere with the chemotherapeutic efficacy of methotrexate.
* **Sulfonamides/Pyrimethamine:** May interfere with folic acid absorption or metabolism.
## Monitoring
* **Hematology:** Monitor Hgb, Hct, MCV, and reticulocyte count.
* **Vitamin B12:** Ensure adequate B12 status before initiating high-dose folic acid to prevent masked neuropathy.
## Clinical Pearls
* The standard therapeutic dose for simple deficiency is often 1 mg/day, despite many over-the-counter supplements containing 400–800 mcg.
* In patients receiving chronic methotrexate for rheumatoid arthritis or psoriasis, folic acid should be administered daily (except on methotrexate dosing days) to reduce mucosal and hematologic toxicity.
* Folic acid is photosensitive; protect storage from light.
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**Educational Disclaimer:** This information is for educational purposes only. Clinical practice guidelines vary by institution and country. Always verify specific dosing, safety precautions, and patient-specific contraindications via current, localized prescribing information or electronic medical resources (e.g., Lexicomp, Micromedex) before prescribing or administering medication.
