Can This Patient Take Augmentin?
Yes, a patient with keflex (cephalexin) and Bactrim allergies can safely take Augmentin (amoxicillin-clavulanate), but ONLY if they have NOT had a previous allergic reaction specifically to amoxicillin or ampicillin. 1
Critical Contraindication You Must Rule Out
Augmentin (amoxicillin) and keflex (cephalexin) share identical R1 side chains, creating significant cross-reactivity risk. 1 This means:
- If the patient's keflex allergy was an immediate-type reaction (urticaria, angioedema, anaphylaxis occurring within 1-6 hours), you should AVOID amoxicillin/Augmentin regardless of severity or timing of the original reaction. 1
- The cross-reactivity between cephalexin and amoxicillin is driven by their identical side chain structure, not the beta-lactam ring. 1, 2
- Patients with positive amoxicillin-IgE have significantly higher rates of cephalexin-IgE positivity (14.00% vs. 2.96%, p<0.01). 3
The Bactrim Allergy is Irrelevant
- Bactrim (sulfamethoxazole-trimethoprim) is a sulfonamide antibiotic with no structural relationship to beta-lactams like Augmentin. [@General Medicine Knowledge]
- The sulfa allergy does not increase risk of reactions to Augmentin and should not influence this decision. [@General Medicine Knowledge]
What You Need to Document Before Prescribing
Determine the exact nature of the keflex reaction: 1
- Immediate-type (within 1-6 hours): urticaria, angioedema, bronchospasm, anaphylaxis → AVOID Augmentin 1
- Delayed-type (after 1 hour): maculopapular rash, delayed urticaria → Consider alternatives 1
- Severe delayed reactions (Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS) → AVOID all beta-lactams including Augmentin 1
Safe Alternative Options
If you cannot use Augmentin due to the keflex allergy:
- Cephalosporins with dissimilar side chains (ceftriaxone, ceftazidime) can be used safely regardless of the keflex reaction type or severity. 1, 4
- Carbapenems can be used in patients with immediate-type cephalexin allergy, regardless of severity or timing. 1
- Fluoroquinolones or macrolides depending on the clinical indication. [@General Medicine Knowledge]
Common Pitfall to Avoid
Do not assume all cephalosporins are the same. 1 The outdated "10% cross-reactivity" myth between penicillins and cephalosporins has been debunked—cross-reactivity is R1 side chain-dependent, not class-dependent. [@4@, 5] Cephalexin specifically shares the same side chain as amoxicillin/ampicillin, making this a high-risk combination. [@3