Azithromycin Dosing for 109 lbs Patient
For a patient weighing 109 lbs (49.5 kg), azithromycin dosing depends entirely on the indication, but for most common adult indications, the standard adult dose of 500 mg on day 1 followed by 250 mg daily on days 2-5 is appropriate.
Weight-Based Dosing Framework
109 lbs converts to approximately 49.5 kg. This weight falls into the adult dosing category for most indications, as the patient exceeds the typical pediatric weight thresholds (45 kg for most guidelines) 1.
Standard Adult Dosing Regimens
Most Common Indications
For community-acquired pneumonia, pharyngitis/tonsillitis (second-line), skin/soft tissue infections, and acute bacterial exacerbations of COPD:
- 500 mg as a single dose on Day 1
- 250 mg once daily on Days 2-5
- Total course: 1.5 grams over 5 days 2
Alternative 3-Day Regimen
For acute bacterial sinusitis and some COPD exacerbations:
- 500 mg once daily for 3 days
- Total course: 1.5 grams 2
Single-Dose Regimens
- Chlamydia/non-gonococcal urethritis: 1 gram single dose 3, 1
- Chancroid: 1 gram single dose 2
- Gonococcal infections: 2 grams single dose (though ceftriaxone is preferred first-line) 2
Pediatric Considerations (If Patient is Adolescent <18 years)
If this 109 lb patient is a child or adolescent under 18 years old, weight-based dosing may be more appropriate for certain indications:
For Pharyngitis/Tonsillitis
- 12 mg/kg once daily for 5 days (maximum 500 mg/day)
- At 49.5 kg: approximately 594 mg daily, so use the adult maximum of 500 mg daily for 5 days 2
- This higher total dose (60 mg/kg total) is superior to lower doses for streptococcal pharyngitis 4, 5
For Respiratory Infections
- 10 mg/kg on Day 1 (maximum 500 mg), then 5 mg/kg on Days 2-5 (maximum 250 mg)
- At 49.5 kg: 495 mg Day 1, then 247.5 mg Days 2-5
- Round to standard adult dosing: 500 mg Day 1,250 mg Days 2-5 2
Critical Dosing Pitfalls
Avoid underdosing for streptococcal pharyngitis: The standard 30 mg/kg total course (10 mg/kg Day 1, then 5 mg/kg Days 2-5) is inferior to penicillin for Group A strep and results in 3 times more bacterial failures 4. For proven streptococcal pharyngitis, either use the higher 60 mg/kg regimen (12 mg/kg daily × 5 days) or choose penicillin instead 6, 4.
Do not adjust for mild-moderate renal impairment: No dose adjustment needed for GFR 10-80 mL/min. Only exercise caution with severe renal impairment (GFR <10 mL/min) 2.
Hepatic impairment: Pharmacokinetics not established; use with caution but no specific dose adjustment available 2.
Drug Interactions to Monitor
- Avoid concurrent aluminum/magnesium antacids (reduces absorption) 6
- Monitor closely with: digoxin, triazolam, ergot alkaloids, warfarin 2
- Contraindicated with: history of cholestatic jaundice or hepatic dysfunction with prior azithromycin use 2