From the Guidelines
For strep A pharyngitis that is resistant to ampicillin but the patient is currently on amoxicillin, you should switch to an alternative antibiotic, such as penicillin G benzathine as a single intramuscular injection of 1.2 million units, or oral cephalosporins like cephalexin (500 mg twice daily for 10 days), as recommended by the Infectious Diseases Society of America in their 2012 guidelines 1. This change is necessary because amoxicillin and ampicillin share the same mechanism of action as they are both aminopenicillins, so resistance to ampicillin typically means resistance to amoxicillin as well. The resistance occurs when bacteria produce beta-lactamase enzymes that break down the beta-lactam ring in these antibiotics. Some key points to consider when selecting an alternative antibiotic include:
- Efficacy: The new antibiotic should be effective against the resistant strain of strep A.
- Safety: The new antibiotic should have a safe profile for the patient.
- Antimicrobial spectrum: A narrow spectrum antibiotic is preferred to minimize the risk of resistance development.
- Dosing schedule: The new antibiotic should have a convenient dosing schedule to improve patient adherence.
- Cost: The new antibiotic should be cost-effective. For patients with penicillin allergy, macrolides like azithromycin (500 mg on day 1, then 250 mg daily for 4 more days) or clindamycin (300 mg three times daily for 10 days) are appropriate alternatives, as suggested in the 2012 guidelines 1. Complete treatment is important to prevent complications like rheumatic fever and to ensure eradication of the bacteria. It's also important to note that the 2012 guidelines from the Infectious Diseases Society of America provide the most recent and highest quality recommendations for the treatment of strep A pharyngitis, and should be followed in clinical practice 1.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Treatment of Strep A Pharyngitis
If a patient is on amoxicillin but resistant to ampicillin, the following options can be considered:
- Cephalosporins: There is low-certainty evidence that cephalosporins may be more effective than penicillin in treating GABHS pharyngitis, particularly in adults 2, 3, 4.
- Macrolides: Macrolides can be used as an alternative to penicillin, but there is low-certainty evidence that they are more effective in treating GABHS pharyngitis 2, 3, 4.
- Carbacephem: Carbacephem may provide better symptom resolution post-treatment in adults and children compared to penicillin, but the evidence is limited 2, 3, 4.
- Azithromycin: Azithromycin may be effective in treating GABHS pharyngitis, particularly in children, but the evidence is limited and there is a higher risk of adverse events compared to amoxicillin 2, 3, 4.
Key Considerations
- The choice of antibiotic should be based on the patient's medical history, allergy status, and local resistance patterns.
- Penicillin can still be considered as a first-line treatment for GABHS pharyngitis, despite the potential for resistance, due to its low cost and narrow spectrum of activity 5, 6.
- Antibiotic treatment should be guided by clinical guidelines and patient-specific factors, rather than empirical treatment alone.
Antibiotic Resistance
- Resistance to ampicillin does not necessarily imply resistance to other antibiotics, such as cephalosporins or macrolides.
- Local resistance patterns should be considered when selecting an antibiotic for treatment.