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# Mefenamic Acid
## Overview
Mefenamic acid is a nonsteroidal anti-inflammatory drug (NSAID) of the fenamate class. It inhibits cyclooxygenase (COX-1 and COX-2) and may also antagonize prostaglandin receptors. Used primarily for mild-to-moderate pain and dysmenorrhea. Onset of analgesia is rapid (within 1–2 hours).
## Primary Indications
- Primary dysmenorrhea
- Mild-to-moderate pain (e.g., headache, dental pain, musculoskeletal pain)
- Fever (less common; other NSAIDs preferred)
## Adult Dosing
- **Immediate release oral (capsules/tablets):** 500 mg as a loading dose, then 250 mg every 6 hours as needed.
- **Maximum daily dose:** 1000 mg (some protocols say 1500 mg short-term; verify locally).
- **Duration:** For dysmenorrhea, start at onset of menses; usually for 2–3 days. Use lowest effective dose for shortest duration.
## Pediatric Dosing
- **Children ≥6 months and adolescents:** 5–10 mg/kg/dose every 6–8 hours as needed.
- **Maximum single dose:** 500 mg.
- **Maximum daily dose:** 20 mg/kg (not to exceed 1000 mg).
- *Note:* Limited pediatric safety data; avoid in children <6 months. Dosing depends on local protocol; verify weight-based calculations.
## Dose Adjustments
- **Renal impairment:** Avoid if CrCl <30 mL/min. Use with caution in mild-to-moderate impairment; reduce dose or interval (e.g., 250 mg every 8–12 hours).
- **Hepatic impairment:** Contraindicated in severe hepatic disease. Use caution; consider avoiding due to increased GI and renal risk.
- **Elderly:** Use lowest effective dose; higher risk of GI bleeding, renal toxicity, and CNS effects.
## Contraindications
- Hypersensitivity to mefenamic acid, aspirin, or any NSAID
- Active peptic ulcer or GI bleeding
- Severe renal or hepatic impairment
- History of asthma, urticaria, or allergic-type reactions after aspirin/NSAID
- Third trimester of pregnancy
- Perioperative pain in coronary artery bypass graft (CABG) surgery
## Adverse Effects
- **Common:** Dyspepsia, nausea, vomiting, diarrhea, headache, dizziness, drowsiness
- **Serious:** GI ulceration/bleeding, renal impairment, hypertension, heart failure, anaphylaxis, hepatic toxicity, blood dyscrasias (rare: hemolytic anemia, agranulocytosis)
- **Note:** Mefenamic acid has a higher incidence of diarrhea than other NSAIDs.
## Key Drug Interactions
- **Anticoagulants (warfarin, DOACs):** Increased bleeding risk
- **Aspirin/other NSAIDs:** Additive GI toxicity
- **ACE inhibitors, ARBs, diuretics:** Reduced antihypertensive/diuretic effect; increased renal risk
- **Lithium, methotrexate:** Increased toxicity of these drugs
- **Corticosteroids:** Increased GI bleeding risk
## Monitoring
- Baseline renal function, liver enzymes, and CBC in prolonged use
- Monitor for signs of GI bleeding, edema, hypertension, and renal impairment
- In pediatric patients, monitor fluid balance and CNS effects (drowsiness, confusion)
## Clinical Pearls
- Rapid onset makes it useful for acute dysmenorrhea; start at onset of pain.
- Avoid use in patients with active diarrhea; may exacerbate.
- Mefenamic acid can interfere with urine tests for bile acids and 5-HIAA.
- Do not use with other NSAIDs or COX-2 inhibitors.
- Discontinue if rash or signs of hemolytic anemia occur.
- Risk of GI harm is similar to other non-selective NSAIDs; consider gastroprotection (PPI) in high-risk patients.
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**Educational Disclaimer:** This information is for educational purposes and does not replace independent verification of current prescribing guidelines, local protocols, and patient-specific factors. Always consult the most recent drug monograph and clinical judgment before prescribing or administering.
