What amoxicillin dose should I give to a 13‑year‑old child weighing about 45 kg using 500 mg capsules?

Amoxicillin Dosing for a 13-Year-Old Weighing 45 kg

Give 500 mg twice daily (every 12 hours) for mild to moderate infections, or 875 mg twice daily for severe infections or high-resistance areas. 1

Weight-Based Dosing Algorithm

Your patient weighs 45 kg, which places them above the 40 kg threshold where adult dosing regimens apply. 1, 2

For Mild to Moderate Infections

• 500 mg every 12 hours is the standard adult dose for ear/nose/throat infections, skin infections, and genitourinary tract infections. 1, 3
• This translates to one 500 mg capsule twice daily—straightforward administration with your available formulation. 1

For Severe Infections or High-Resistance Areas

• 875 mg every 12 hours is indicated for severe respiratory infections, community-acquired pneumonia, or areas with >10% penicillin-resistant Streptococcus pneumoniae. 1, 2
• Since you only have 500 mg capsules, you would need to give two 500 mg capsules (1000 mg) twice daily to approximate this dose—this slightly exceeds the 875 mg target but remains within safe limits (maximum single dose is 2000 mg). 1, 2

Indication-Specific Guidance

Community-Acquired Pneumonia or Lower Respiratory Tract Infection

• Use the high-dose regimen: 1000 mg (two 500 mg capsules) twice daily for 10 days. 1, 2
• This provides adequate coverage for penicillin-resistant S. pneumoniae and typical respiratory pathogens. 1, 4

Group A Streptococcal Pharyngitis (Strep Throat)

• The recommended dose is 50–75 mg/kg/day divided into two doses, which equals 1125–1687 mg per dose for a 45 kg patient. 1, 2
• However, the maximum single dose is capped at 1000 mg, so give 1000 mg (two 500 mg capsules) twice daily for 10 days. 1, 2
• The full 10-day course is mandatory to prevent acute rheumatic fever, even if symptoms resolve earlier. 1, 2

Acute Bacterial Sinusitis

• For patients ≥2 years without recent antibiotic exposure or daycare attendance: 500 mg twice daily. 2
• For recent antibiotic use (within 30 days), daycare attendance, or age <2 years: 1000 mg twice daily (high-dose regimen). 2

Ear/Nose/Throat or Skin Infections (Uncomplicated)

• 500 mg twice daily for 7–10 days is appropriate for mild to moderate infections. 1, 3

Treatment Duration

• Respiratory infections (including pneumonia): 10 days minimum. 1, 2
• Group A Streptococcal infections: Exactly 10 days to prevent rheumatic fever. 1, 2
• Other infections: 7–10 days, continuing for at least 48–72 hours after symptom resolution. 1, 3

When to Switch to Amoxicillin-Clavulanate

• If the patient has recent antibiotic exposure (within 30 days), suspect β-lactamase-producing organisms (H. influenzae, M. catarrhalis) and consider switching to amoxicillin-clavulanate 90 mg/kg/day of the amoxicillin component. 2, 5
• If no clinical improvement occurs within 48–72 hours, reassess and consider amoxicillin-clavulanate or alternative antibiotics. 1, 2

Critical Administration Details

• Take at the start of a meal to minimize gastrointestinal side effects. 3
• Complete the full prescribed course even if symptoms improve before completion. 1, 2
• Expect clinical improvement within 48–72 hours; if not, reevaluate the diagnosis and consider treatment failure. 1, 2

Common Pitfalls to Avoid

• Do not underdose: A 45 kg adolescent requires adult dosing, not pediatric weight-based calculations that would yield lower doses. 1, 2
• Do not use 250 mg capsules if available—they would require four capsules per dose for high-dose regimens, increasing pill burden and reducing adherence. 1
• Do not stop early: Premature discontinuation, especially in streptococcal infections, risks rheumatic fever. 1, 2
• Twice-daily dosing improves adherence compared to three-times-daily regimens, making it the preferred schedule. 1

Maximum Dosing Limits

• Maximum single dose: 2000 mg per administration (your 1000 mg twice-daily regimen is well within this limit). 2
• Maximum daily dose: 4000 mg per day. 1, 2

Monitoring and Follow-Up

• Fever and pain should resolve within 24–48 hours for most bacterial infections. 2
• If symptoms persist or worsen after 72 hours, consider atypical pathogens, resistant organisms, or alternative diagnoses. 1, 2
• Gastrointestinal disturbances (diarrhea, nausea) are the most common adverse effects; rash and hypersensitivity reactions may also occur. 1, 2