What is the appropriate amoxicillin dosage for a 9-year-old child weighing 55 kg with streptococcal pharyngitis?

Amoxicillin Dosing for Streptococcal Pharyngitis in a 9-Year-Old, 55 kg Child

For this 9-year-old child weighing 55 kg with strep throat, prescribe amoxicillin 1000 mg once daily for 10 days, or alternatively 500 mg twice daily for 10 days. 1

Weight-Based Dosing Calculation

• The recommended dose is 50 mg/kg once daily (maximum 1000 mg) or 25 mg/kg twice daily (maximum 500 mg per dose) for 10 days. 1
• For this 55 kg child, the calculated dose would be 2750 mg once daily or 1375 mg twice daily, but both exceed the maximum recommended doses of 1000 mg once daily or 500 mg twice daily. 1
• Therefore, use the maximum adult dose: 1000 mg once daily OR 500 mg twice daily for 10 days. 2, 1

Why This Dosing Regimen

• Once-daily amoxicillin at 1000 mg has been validated as non-inferior to twice-daily dosing in multiple randomized controlled trials, with bacteriologic failure rates of 20.1% vs 15.5% at 14-21 days (difference 4.53%; 90% CI -0.6 to 9.7), meeting non-inferiority criteria. 3
• The once-daily regimen may improve adherence compared to multiple daily doses, with compliance rates exceeding 95% in clinical trials. 3
• Amoxicillin remains the drug of choice due to proven efficacy, narrow spectrum, excellent safety profile, low cost, and the complete absence of documented penicillin resistance in Group A Streptococcus worldwide. 2

Critical Treatment Duration Requirement

• The full 10-day course is mandatory to achieve maximal pharyngeal eradication of Group A Streptococcus and prevent acute rheumatic fever, even if symptoms resolve within 3-4 days. 2, 1
• Shortening the course by even a few days results in appreciable increases in treatment failure rates and rheumatic fever risk. 2
• The primary goal is not just symptomatic relief but prevention of acute rheumatic fever, which requires adequate bacterial eradication. 2

Alternative Regimens for Penicillin Allergy

• For non-immediate penicillin allergy (e.g., mild delayed rash): First-generation cephalosporins such as cephalexin 500 mg twice daily for 10 days, with only 0.1% cross-reactivity risk. 2
• For immediate/anaphylactic penicillin allergy (hives, angioedema, bronchospasm within 1 hour): Clindamycin 300 mg three times daily for 10 days (approximately 1% resistance rate in the US). 2
• Azithromycin 500 mg once daily for 5 days is acceptable but has 5-8% macrolide resistance rates in the United States. 2

Common Pitfalls to Avoid

• Do not prescribe doses below the maximum adult dose for this weight – the child has reached adult dosing thresholds at 55 kg. 1
• Do not shorten the treatment course below 10 days despite clinical improvement, as this dramatically increases treatment failure and rheumatic fever risk. 2, 1
• Do not use trimethoprim-sulfamethoxazole (Bactrim) – it fails to eradicate Group A Streptococcus in 20-25% of cases. 2

Expected Clinical Response

• Patients typically become non-contagious after 24 hours of antibiotic therapy. 1
• Symptoms usually resolve within 3-4 days, but this does not indicate completion of bacterial eradication. 2
• Routine post-treatment throat cultures are not recommended for asymptomatic patients who have completed therapy. 2