What is the appropriate amoxicillin dose for a 17-year-old girl weighing 120 lb?

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Last updated: September 11, 2026 • View editorial policy

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Amoxicillin Dosing for a 17-Year-Old, 120 lb Patient

For a 17-year-old girl weighing 120 lb (54.5 kg), the appropriate amoxicillin dose depends on the specific infection being treated, but for most common respiratory infections, prescribe 500 mg twice daily or 250 mg four times daily for 10 days.

Weight-Based Dosing Considerations

At 120 lb (54.5 kg), this patient exceeds 40 kg, placing her in the adult dosing category according to FDA labeling 1. However, the specific indication matters significantly:

For Group A Streptococcal Pharyngitis (Strep Throat)

  • Standard dose: 500 mg twice daily for 10 days 2
  • Alternative: 250 mg four times daily for 10 days
  • This represents strong, high-quality evidence from IDSA guidelines

For Community-Acquired Pneumonia

The dosing is more nuanced:

  • For mild-moderate infections: 500 mg every 12 hours 1
  • For severe infections or suspected resistant organisms: 875 mg every 12 hours 1

The pediatric CAP guidelines recommend 90 mg/kg/day in 2 doses for children, which would calculate to approximately 2,450 mg/day (1,225 mg twice daily) for this 54.5 kg patient 3. However, this creates a critical dosing dilemma that the evidence highlights.

The Weight-Based Dosing Paradox

A significant issue exists in adolescent dosing: Research demonstrates that larger children systematically receive lower mg/kg doses because clinicians are reluctant to exceed "adult maximum doses" 4, 5. Children reaching 60 kg often receive less than 40 mg/kg/day, well below the recommended 80-90 mg/kg/day for resistant pneumococcal infections.

Practical Dosing Algorithm

For this 17-year-old patient:

  1. If treating strep pharyngitis or mild skin/soft tissue infection:

    • 500 mg twice daily × 10 days 2

  2. If treating suspected pneumococcal pneumonia with local resistance concerns:

    • Consider 875 mg twice daily (approximately 32 mg/kg/day) 1
    • This balances efficacy against resistance while staying within FDA-approved adult dosing

  3. If treating severe pneumonia requiring high-dose therapy:

    • 875 mg twice daily is the maximum oral formulation available
    • If inadequate response, consider parenteral therapy rather than exceeding oral dosing limits

Important Caveats

Do not exceed 875 mg per dose or 1,750 mg/day for oral amoxicillin in routine outpatient management, as this represents the maximum FDA-approved formulation strength 1. The evidence shows that while weight-based dosing of 80-90 mg/kg/day is ideal for resistant organisms, practical limitations exist with available formulations.

For infections requiring higher exposure: If true high-dose therapy (80-90 mg/kg/day) is needed based on local resistance patterns, this would require approximately 4,400 mg/day for this patient—a dose not achievable with standard oral formulations and not supported by FDA labeling for routine use 1.

Duration matters: Ensure 10-day treatment for streptococcal infections to prevent rheumatic fever 2, 1. For pneumonia, treatment should continue 48-72 hours beyond symptom resolution 1.

The key takeaway: adolescents in this weight range fall into a dosing gap where pediatric weight-based recommendations exceed adult maximum doses, requiring clinical judgment based on infection severity and local resistance patterns 4, 5.

References

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.