Cephalexin Use in Patients with Amoxicillin and Ampicillin Allergies

Understanding Cross-Reactivity Risk

Cephalexin shares identical R1 side chains with amoxicillin and ampicillin, which is the primary mechanism for cross-reactivity between these antibiotics 1, 2
Cross-reactivity between penicillins and cephalosporins is largely based on these R1 side chains, with identical side chains posing the highest risk 1

Patients with immediate-type allergies to amoxicillin or ampicillin should avoid cephalexin regardless of when the reaction occurred 3, 4
Patients with delayed-type allergies to amoxicillin or ampicillin that occurred within the past year should avoid cephalexin 3, 5
If the delayed-type reaction occurred more than 1 year ago, cephalexin might be considered with caution 3, 5

Cephalosporins with dissimilar R1 side chains to amoxicillin/ampicillin can be used safely 6, 3
Cefazolin is particularly safe as it does not share side chains with any currently available penicillins 5, 3
For conditions like Group A streptococcal pharyngitis, alternatives include Clindamycin (7 mg/kg three times daily, max 300 mg per dose for 10 days), Clarithromycin (7.5 mg/kg twice daily, max 250 mg per dose for 10 days), and Azithromycin (12 mg/kg once daily, max 500 mg for 5 days) 7

If cephalexin must be used in a patient with non-severe, immediate-type reactions to amoxicillin/ampicillin that occurred >5 years ago, consider administration in a controlled setting with monitoring 3, 8