What is the appropriate naproxen dose for a 16‑year‑old weighing 41.9 kg according to the BNFC?

Naproxen Dosing for a 16-Year-Old Weighing 41.9 kg

For this 16-year-old patient weighing 41.9 kg, administer naproxen 250 mg twice daily (every 12 hours) as the starting dose, which can be adjusted up to 500 mg twice daily based on clinical response. 1

Weight-Based Dosing Considerations

• This patient falls into an important transitional weight category where both pediatric weight-based dosing (approximately 10 mg/kg given in 2 divided doses) and adult fixed dosing may be considered 1
• The calculated pediatric dose would be approximately 210 mg twice daily (5 mg/kg × 41.9 kg), but naproxen tablets are not well suited to precise weight-based dosing in this weight range 1
• Adult fixed-dose regimens become standard for patients at or above 40 kg for most medications, making the 250 mg twice daily starting dose appropriate 2

Recommended Dosing Regimen

• Start with 250 mg twice daily (every 12 hours) for rheumatic conditions or chronic pain 1
• The dose may be increased to 375 mg or 500 mg twice daily depending on clinical response during long-term administration 1
• The morning and evening doses do not have to be equal in size, and administration more frequently than twice daily is not necessary 1

For Acute Pain Conditions

• If treating acute pain, primary dysmenorrhea, or acute tendonitis/bursitis, consider starting with 500 mg, followed by 500 mg every 12 hours or 250 mg every 6-8 hours as required 1
• The initial total daily dose should not exceed 1250 mg, and thereafter the total daily dose should not exceed 1000 mg 1

Critical Safety Considerations

• Use the lowest effective dose for the shortest duration consistent with treatment goals 1
• At this weight (41.9 kg), the patient is close to the adult dosing threshold but may have increased unbound plasma fraction compared to heavier adults, requiring careful monitoring 1, 3
• Naproxen is contraindicated in patients with moderate to severe renal impairment (creatinine clearance <30 mL/min) 1
• Monitor for gastrointestinal, cardiovascular, and renal adverse effects, which are common with NSAIDs 1, 4

Common Pitfalls to Avoid

• Do not attempt to use naproxen oral suspension for precise weight-based dosing in adolescents—tablets are the appropriate formulation for this age group 1
• Avoid combining naproxen with other NSAIDs without dose adjustment, as this increases toxicity risk 5
• Be aware that different naproxen formulations (tablets, suspension, delayed-release tablets, naproxen sodium) have pharmacokinetic differences that may affect onset of action and should not be considered bioequivalent 1
• In this transitional weight range, starting at the lower end of the adult dosing range (250 mg twice daily) is prudent, with upward titration based on response 1, 3