What is the dosage of Keflex (cephalexin) for treating streptococcal pharyngitis?

Keflex (Cephalexin) for Streptococcal Pharyngitis

For adults with strep throat, cephalexin 500 mg orally every 12 hours for 10 days is the FDA-approved dosing regimen, but this should only be used in penicillin-allergic patients without anaphylactic reactions—penicillin or amoxicillin remains the drug of choice for all non-allergic patients. 1

When to Use Cephalexin

Cephalexin is reserved for penicillin-allergic patients with non-immediate (non-anaphylactic) reactions only. 2, 3

• First-generation cephalosporins like cephalexin are the preferred alternative for patients with non-immediate penicillin allergies due to their narrow spectrum, proven efficacy, and low cost 2, 3
• Patients with immediate hypersensitivity reactions (anaphylaxis, angioedema, respiratory distress, or urticaria within 1 hour) must avoid all cephalosporins due to up to 10% cross-reactivity risk 2, 4
• Penicillin or amoxicillin remains the treatment of choice for non-allergic patients based on proven efficacy, safety, narrow spectrum, and low cost 2

Specific Dosing Regimens

Adults

• 500 mg orally every 12 hours for 10 days 1
• The FDA label explicitly states this dosing for streptococcal pharyngitis 1
• Daily doses should remain within 1 to 4 grams per day 1

Pediatric Patients

• 20 mg/kg per dose twice daily (maximum 500 mg per dose) for 10 days 3
• Alternative: 25 to 50 mg/kg/day in divided doses 1
• For streptococcal pharyngitis in patients over 1 year of age, the total daily dose may be divided and administered every 12 hours 1

Critical Treatment Duration

The full 10-day course is mandatory regardless of symptom resolution. 2, 3

• All oral antibiotics except azithromycin must be administered for 10 days to achieve maximal pharyngeal eradication of Group A Streptococcus 2
• Shortening the course below 10 days results in appreciable increases in treatment failure rates and risk of acute rheumatic fever 3
• Therapy can be safely postponed up to 9 days after symptom onset and still prevent acute rheumatic fever 3

Why Not First-Line?

Penicillin has never developed resistance in Group A Streptococcus anywhere in the world. 2

• Penicillin remains the treatment of choice because of proven efficacy, safety, narrow spectrum, and low cost 2
• Cephalosporins have a broader spectrum than penicillin, which unnecessarily increases selection pressure for antibiotic-resistant flora 3
• Amoxicillin is often used in place of penicillin V for young children due to better taste acceptance, with equal efficacy 2

Common Pitfalls to Avoid

• Do not use cephalexin in patients with immediate/anaphylactic penicillin reactions due to the 10% cross-reactivity risk 2, 4
• Do not prescribe cephalexin as first-line therapy when penicillin or amoxicillin can be used—this unnecessarily broadens antibiotic spectrum and increases cost 3
• Do not shorten the treatment course below 10 days despite clinical improvement, as this increases treatment failure rates and risk of acute rheumatic fever 3
• Do not use broad-spectrum cephalosporins when narrow-spectrum first-generation agents are appropriate 3

Alternative Options if Cephalexin Cannot Be Used

For Immediate/Anaphylactic Penicillin Allergy:

• Clindamycin 7 mg/kg per dose three times daily (maximum 300 mg per dose) for 10 days is the preferred choice 3, 4
• Clindamycin has approximately 1% resistance rate among Group A Streptococcus in the United States 3, 4
• Azithromycin 12 mg/kg once daily (maximum 500 mg) for 5 days is an acceptable alternative 3
• Azithromycin has 5-8% macrolide resistance in the United States and varies geographically 3, 4
• Clarithromycin 7.5 mg/kg per dose twice daily (maximum 250 mg per dose) for 10 days is also acceptable 3

Adjunctive Therapy

• Acetaminophen or NSAIDs (ibuprofen) should be considered for moderate to severe symptoms or control of high fever 3
• Aspirin must be avoided in children due to the risk of Reye syndrome 3
• Corticosteroids are not recommended as adjunctive therapy 3