Non-allergic adverse effectNot an allergy
Nausea, diarrhea, headache, yeast infection, family history onlyReconcile the label. These histories do not establish immune cross-sensitivity.
Drug allergy · Point-of-care reference
Beta-lactam Cross-Sensitivity
Start with the reaction phenotype, then use R1 side-chain relationships. Cephalosporin generation alone is not a reliable cross-reactivity rule.
Evidence recordRestricted clinical scopeVersion 0.2 · Updated July 21, 2026
First question
Was this actually an allergy—and what phenotype?
Do not use a side-chain chart to clear a patient with SJS/TEN, DRESS, AGEP, immune cytopenia, interstitial nephritis, hepatitis, or another severe delayed/organ-specific reaction. Confirm the culprit, timing, symptoms, treatment, and subsequent tolerated beta-lactams.
One-page beta-lactam cross-referenceUS Letter landscape · reaction triage · R1 side-chain groups · bedside exceptions
Open / print PDF Remote, benign, or unknown historyUse after assessment
Remote childhood rash, vague history, nonurticarial rash without systemic featuresUse a validated history pathway. Many patients are candidates for direct challenge or routine beta-lactam use under local policy.
Immediate / IgE-typeDo not use alone
Urticaria, angioedema, bronchospasm, hypotension, anaphylaxisConfirm culprit and timing; use current allergy guidance and a structurally dissimilar agent or supervised evaluation as appropriate.
Severe delayed immune reactionDo not use alone
SJS/TEN, DRESS, AGEP, serum sickness, hemolytic anemia, interstitial nephritis, hepatitisStop. A side-chain chart is not a clearance tool for severe delayed or organ-specific reactions. Obtain allergy/ID/specialist guidance.
6 of 6 clinically useful relationship groups shown
| Relationship group | Drugs to connect | Why they are grouped | Bedside meaning | Evidence anchor |
|---|---|---|---|---|
| Aminopenicillin ↔ aminocephalosporin | AmoxicillinAmpicillinCefadroxilCefaclorCephalexinCefprozil | Highest clinically important penicillin–cephalosporin R1 similarity cluster | Do not treat every cephalosporin as equivalent. Amoxicillin aligns most closely with cefadroxil/cefprozil; ampicillin aligns most closely with cefaclor/cephalexin. | Romano 2019 meta-analysis · Chaudhry 2019 side-chain review |
| Methoxyimino cephalosporins | CefuroximeCefotaximeCeftriaxoneCefepimeCefpodoxime | Related R1 features within the cephalosporin class | Cross-reactivity may occur within this cluster. A prior cephalosporin reaction should be assessed by the culprit drug and side chain—not by generation alone. | Chaudhry 2019 side-chain review · 2022 Drug Allergy Practice Parameter |
| Ceftazidime ↔ aztreonam | CeftazidimeAztreonam | Shared R1 side chain | Aztreonam generally has minimal clinical cross-reactivity with other beta-lactams, but ceftazidime is the key exception to check. | CSACI position statement 2020 · Chaudhry 2019 side-chain review |
| Cefazolin | Cefazolin | Unique R1 side chain among commonly used beta-lactams | Penicillin allergy alone does not predict cefazolin allergy. The 2022 practice parameter supports cefazolin for most penicillin-allergic patients, including many with anaphylaxis histories, but not as a shortcut for severe delayed reactions or known cefazolin allergy. | 2022 Drug Allergy Practice Parameter · CSACI position statement 2020 |
| Carbapenems | ErtapenemImipenem/cilastatinMeropenem | Low observed cross-reactivity with proven penicillin allergy | A 2019 meta-analysis estimated 0.87% carbapenem cross-reactivity in patients with proven penicillin allergy. The 2022 practice parameter permits routine carbapenem administration in many penicillin/cephalosporin allergy histories. | Romano 2019 meta-analysis · 2022 Drug Allergy Practice Parameter |
| Low-similarity cephalosporins | Structurally dissimilar cephalosporin | Risk follows R1 similarity more than cephalosporin generation | In proven penicillin allergy, pooled risk was 2.11% for low-similarity cephalosporins versus 16.45% for identical-side-chain aminocephalosporins. Use the exact culprit and formulation—not a class-wide percentage. | Romano 2019 meta-analysis |
In patients with proven penicillin allergy, a 2019 meta-analysis found cephalosporin cross-reactivity of 16.45% with identical R1 side chains, 5.60% with intermediate similarity, and 2.11% with low similarity; carbapenem cross-reactivity was 0.87%.
Documented tolerance, supervised challenge, skin testing when indicated, and the institution's allergy pathway determine whether an allergy label should change.
Evidence packet
What this tool is based on
This versioned record keeps the citations, equation or decision rule, units, test coverage, intended population, and exclusions attached to the tool.
How evidence review works →Current U.S. practice parameter · GuidanceDrug allergy: A 2022 practice parameter update.J Allergy Clin Immunol. 2022;150(6):1333–1393. PMID: 36122788 ↗Cross-reactivity meta-analysis · Evidence reviewRomano et al. Cross-reactivity of cephalosporins and carbapenems in penicillin-allergic patients.J Allergy Clin Immunol Pract. 2019;7(8):2722–2738.e5. PMID: 31170539 ↗R1 side-chain map · Evidence reviewChaudhry et al. Cephalosporins: A focus on side chains and beta-lactam cross-reactivity.Pharmacy (Basel). 2019;7(3):103. doi:10.3390/pharmacy7030103 ↗Practical allergy guidance · GuidanceCSACI position statement: Practical guide for evaluation and management of beta-lactam allergy.Allergy Asthma Clin Immunol. 2020;16:95. doi:10.1186/s13223-020-00494-2 ↗
Intended population
Clinicians assessing a reported beta-lactam allergy while using a current institutional drug-allergy pathway and the exact culprit/reaction history.
Equation record
Reaction-phenotype triage plus a simplified R1 side-chain relationship map. Cross-reactivity estimates are shown only with the allergy-verification and structural-similarity populations from which they were derived.
Unit handling
Reference table only; no patient-specific calculation. Percentages are pooled study estimates and retain their source population and confidence limitations.
Test coverage
Search aliases, phenotype warnings, aminopenicillin/aminocephalosporin cluster, cefazolin uniqueness, ceftazidime/aztreonam exception, methoxyimino cluster, carbapenem estimate, empty results, and one-page PDF integrity.
Do not skip
Limitations and exclusions
- This chart does not diagnose, exclude, or de-label a drug allergy.
- Do not use the side-chain map alone for SJS/TEN, DRESS, AGEP, organ-specific immune injury, immune cytopenia, serum sickness, or another severe delayed reaction.
- A drug tolerated after the index reaction can materially change the assessment and should be documented.
- Brand combinations and newer beta-lactam/beta-lactamase inhibitor products require checking every beta-lactam component.
- Cross-reactivity estimates from proven allergy cohorts should not be presented as an individual patient's probability.
- Challenge, skin testing, desensitization, observation, and emergency-readiness procedures must follow current allergy guidance and institutional policy.
One clearly labeled, contextually relevant sponsor may support this reference area. Sponsorship will never interrupt inputs, obscure results, influence formulas, or imply clinical endorsement.
About this tool
Beta-Lactam Allergy Cross-Reactivity Chart
Use this beta-lactam cross-reactivity reference to review the reported reaction phenotype and relevant R1 side-chain relationships rather than relying on the class name alone.
Allergy assessment is patient- and reaction-specific. Current guidance, drug structure, reaction timing and severity, testing history, and local pathways govern the final decision.