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# Folic Acid
## Overview
Folic acid (vitamin B9) is a water-soluble B-complex vitamin essential for DNA synthesis, repair, and erythropoiesis. Exogenous supplementation is required as humans cannot synthesize folate de novo.
## Primary Indications
* Treatment and prophylaxis of folate deficiency anemia.
* Prevention of neural tube defects (NTDs) during pregnancy.
* Adjunct therapy during methotrexate or sulfonamide treatment to reduce hematologic and GI toxicity.
## Adult Dosing
* **Deficiency/Anemia:** 1 mg daily. May increase to 5 mg/day in malabsorption syndromes.
* **Pregnancy (Prevention of NTDs):** 0.4–0.8 mg daily, starting at least 1 month before conception and continuing through the first trimester. High-risk patients (history of NTD-affected pregnancy): 4 mg daily.
* **Methotrexate-induced toxicity:** 1–5 mg daily (non-dose day).
## Pediatric Dosing
* **Deficiency (Infants):** 0.1 mg daily.
* **Deficiency (Children 1–11 years):** 0.2–1 mg daily.
* **Deficiency (Children ≥12 years):** 1 mg daily.
* **Maintenance:** 0.1–0.4 mg daily depending on age.
## Dose Adjustments
* **Renal/Hepatic Impairment:** No specific dosage adjustments established.
* **Malabsorption/Chronic Hemolytic Anemia:** May require higher doses (up to 5 mg daily); monitor status closely.
## Contraindications
* Hypersensitivity to folic acid.
* **Crucial:** Do not use as monotherapy for pernicious anemia (B12 deficiency) or other megaloblastic anemias where B12 deficiency is present, as it can resolve hematologic symptoms while allowing irreversible neurological damage to progress.
## Adverse Effects
* Generally rare; includes allergic reactions (rash, pruritus, bronchospasm).
* High doses (>15 mg/day) may cause GI distress, irritability, or sleep disturbances.
## Key Drug Interactions
* **Methotrexate:** Folic acid may decrease the efficacy of methotrexate in oncology; dosing must be coordinated carefully.
* **Anticonvulsants (Phenytoin, Phenobarbital, Primidone):** Folate supplementation may decrease serum concentrations of these agents, potentially reducing seizure control.
* **Sulfonamides/Trimethoprim:** May decrease folic acid absorption or metabolism.
## Monitoring
* **Baseline:** Complete blood count (CBC), serum B12 level to rule out B12 deficiency.
* **Ongoing:** Hemoglobin/hematocrit, MCV, and resolution of deficiency symptoms.
## Clinical Pearls
* Ensure vitamin B12 levels are verified before high-dose folic acid therapy to prevent masking subacute combined degeneration of the spinal cord.
* Bioavailability is high; food does not significantly affect absorption.
* For patients on chronic methotrexate, ensure folic acid is administered on "off days" only, per institutional protocol, to avoid interfering with therapeutic effectiveness.
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*Disclaimer: This information is for educational purposes and does not replace professional medical judgment. Always verify dosages, contraindications, and drug interactions against institutional protocols and the most current prescribing information (e.g., Lexicomp, Micromedex) before prescribing or administering medication.*
