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# Folic Acid
## Overview
Folic acid (Vitamin B9) is a water-soluble B-vitamin essential for DNA synthesis, repair, and erythropoiesis. Exogenous supplementation is required as the human body cannot synthesize folate.
## Primary Indications
* Treatment/prevention of folate deficiency anemia.
* Prevention of neural tube defects (NTDs) in pregnancy.
* Adjunct therapy for methotrexate-induced toxicity.
* Reduction of hyperhomocysteinemia.
## Adult Dosing
* **Folate Deficiency:** 0.4–1 mg daily. Severe cases may require up to 5 mg daily.
* **Prevention of NTDs:** 0.4–0.8 mg daily, ideally starting at least one month preconception and continuing through the first trimester. High-risk patients (history of NTD-affected pregnancy) may require 4 mg daily (consult clinical guidelines, e.g., ACOG).
* **Methotrexate Toxicity:** 1–5 mg daily (non-methotrexate days) or 5–10 mg weekly following methotrexate doses, per institutional protocol.
## Pediatric Dosing
* **Maintenance:** 0.1–0.3 mg daily (infants); 0.4 mg daily (children/adolescents).
* **Folate Deficiency:** 0.5–1 mg daily.
* **Methotrexate Toxicity (Pediatric):** 1–5 mg/day on non-methotrexate days; dosing is highly variable based on oncology protocols.
## Dose Adjustments
* **Renal/Hepatic Impairment:** No specific adjustment necessary.
* **Conditions:** Patients with suspected vitamin B12 deficiency must be evaluated before initiating high-dose folic acid, as folic acid can mask B12-deficient megaloblastic anemia while allowing irreversible neurological damage to progress.
## Contraindications
* Hypersensitivity to folic acid.
* Undiagnosed anemia (specifically vitamin B12 deficiency).
* Pernicious anemia.
## Adverse Effects
* Generally well-tolerated.
* Rare: Allergic reactions (rash, pruritus, dyspnea).
* Very high doses (15 mg/day) may cause GI distress, sleep disturbances, or malaise.
## Key Drug Interactions
* **Anticonvulsants (Phenytoin, Carbamazepine, Valproate):** Folic acid may decrease serum concentrations of these agents, potentially reducing seizure control.
* **Methotrexate:** Folic acid may antagonize the therapeutic effects of methotrexate if administered concurrently for chemotherapy; use only as instructed by oncology protocols.
* **Sulfonamides/Trimethoprim:** May interfere with folate metabolism.
## Monitoring
* Serum folate levels (though rarely needed in stable patients).
* Complete Blood Count (CBC) and mean corpuscular volume (MCV) to monitor resolution of megaloblastic anemia.
* Neurological status in patients at risk for B12 deficiency.
## Clinical Pearls
* **B12 Link:** Always exclude B12 deficiency before using folic acid doses >1 mg/day.
* **Bioavailability:** More stable and better absorbed than natural dietary folates.
* **Pregnancy:** USPSTF recommends all women planning or capable of pregnancy take a daily supplement of 0.4–0.8 mg.
* **Local Protocols:** Oncology and hematology dosing for folic acid varies widely; always verify specific clinical pathways for therapeutic drug monitoring.
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**Educational Disclaimer:** This information is for educational purposes only. Always consult current, peer-reviewed clinical guidelines, institutional prescribing policies, and the official FDA-approved labeling (Package Insert) before prescribing or administering medication.
