Can amoxicillin be used to treat pharyngitis in a patient allergic to Dilantin (phenytoin)?

Amoxicillin Use for Pharyngitis in Patients with Dilantin Allergy

Amoxicillin can safely be used to treat pharyngitis in patients with a Dilantin (phenytoin) allergy, as there is no cross-reactivity between these medications. A Dilantin allergy does not contraindicate the use of amoxicillin for streptococcal pharyngitis.

Antibiotic Selection for Pharyngitis

First-line Options

• Penicillin V remains the drug of choice for group A streptococcal pharyngitis 1
• Amoxicillin is an effective alternative with comparable efficacy:
  • Dosage: 50 mg/kg once daily (maximum 1000 mg) or 25 mg/kg twice daily (maximum 500 mg per dose) for 10 days 2
  • Once-daily dosing has been shown to be as effective as multiple daily doses, potentially improving compliance 3

Allergies and Alternative Options

• For patients with penicillin allergy (not relevant to Dilantin allergy):
  • Clindamycin is recommended for those allergic to both penicillin and cephalosporins 2
  • Macrolides (azithromycin, clarithromycin, erythromycin) are alternatives but have increasing resistance rates (5-8% in US) 2

Clinical Decision-Making

1. Confirm streptococcal etiology:

   • Use Centor criteria (fever, tonsillar exudates, no cough, tender anterior cervical lymphadenopathy) 4
   • Test patients with ≥2 criteria using rapid antigen detection test 4
   • Limit antibiotic therapy to patients with positive test results 4

2. Treatment duration:

   • Standard 10-day course is recommended for complete eradication 2
   • Studies show 10-day treatment is more effective than shorter courses 2

Efficacy of Amoxicillin

Amoxicillin has demonstrated excellent efficacy for streptococcal pharyngitis:

• In comparative studies, once-daily amoxicillin (750 mg) showed similar or better bacteriologic cure rates compared to penicillin V given three times daily 3
• Higher dose amoxicillin (40 mg/kg/day) demonstrated superior clinical cure rates (87.9% vs 70.9%) and bacteriologic cure rates (79.3% vs 54.5%) compared to standard-dose penicillin V 5

Important Considerations

• Monitoring for allergic reactions: While not related to Dilantin allergy, patients should be counseled about potential allergic reactions to amoxicillin as it is a penicillin-class drug 6
• Adverse effects: Monitor for diarrhea, which is a common side effect of antibiotics 6
• Patient education: Advise patients to complete the full 10-day course to prevent treatment failure and antimicrobial resistance 6

Practical Points

• Amoxicillin can be administered every 8 or 12 hours depending on the prescribed dose 6
• For children, once-daily amoxicillin (750 mg) has shown comparable efficacy to three-times-daily penicillin V 3
• Amoxicillin suspensions should preferably be refrigerated and shaken well before each use 6

In conclusion, there is no cross-reactivity between Dilantin (phenytoin) and amoxicillin, making amoxicillin a safe and effective choice for treating streptococcal pharyngitis in patients with Dilantin allergy.