Amoxicillin Dosing for Strep Throat in a 15-Year-Old Weighing 50 kg
For a 15-year-old patient weighing 50 kg with streptococcal pharyngitis, the recommended dose of amoxicillin is 50 mg/kg once daily (maximum 1,000 mg) or 25 mg/kg twice daily (maximum 500 mg per dose) for 10 days. 1, 2, 3
Dosing Options
Preferred Regimen:
- Once-daily dosing: 50 mg/kg/day = 50 × 50 kg = 2,500 mg, but capped at maximum 1,000 mg once daily for 10 days
- Twice-daily dosing: 25 mg/kg/dose = 25 × 50 kg = 1,250 mg per day, but capped at 500 mg per dose twice daily for 10 days
Since the calculated dose exceeds the maximum recommended dose in both cases, the appropriate dosing would be:
- 1,000 mg once daily for 10 days, OR
- 500 mg twice daily for 10 days
Duration of Treatment
The duration of treatment for streptococcal pharyngitis should be 10 days, regardless of symptom resolution 1, 2. This duration is critical to prevent acute rheumatic fever, a serious complication of inadequately treated strep throat 3.
Alternative Regimens
If the patient has a penicillin allergy, alternative treatments include:
- Cephalexin: 20 mg/kg twice daily (maximum 500 mg per dose) for 10 days
- Clindamycin: 7 mg/kg three times daily (maximum 300 mg per dose) for 10 days
- Azithromycin: 12 mg/kg once daily (maximum 500 mg) for 5 days 1, 2
Clinical Considerations
Efficacy
- Once-daily amoxicillin has been shown to be non-inferior to twice-daily dosing in multiple studies 4, 5, with comparable bacteriologic eradication rates.
- Amoxicillin has demonstrated superior bacteriologic cure rates compared to conventional penicillin V dosing in some studies (79.3% vs 54.5%) 6.
Administration
- Amoxicillin should be taken at the start of a meal to minimize gastrointestinal intolerance 3.
- The patient is considered non-contagious after 24 hours of antibiotic therapy 2.
Follow-up
- Routine post-treatment throat cultures are not recommended 2.
- If symptoms persist or worsen after 5 days of treatment, consider:
- Non-compliance with the prescribed regimen
- Treatment failure
- New infection
- Viral pharyngitis in a streptococcal carrier 2
Important Precautions
- Complete the full 10-day course even if symptoms resolve quickly, to prevent complications and recurrence 2, 3.
- Monitor for adverse effects, which may include diarrhea, nausea, and rash.
- If allergic reactions occur (seen in approximately 0.9% of patients), discontinue treatment immediately and seek medical attention 5.
The once-daily dosing regimen may improve adherence compared to multiple daily doses, which is particularly important in adolescent patients 4, 5.