Β-lactam antibiotic reactions to any β-lactam antibiotic may occur in up to 10% of patients receiving that agent (a small fraction of which are truly IgE-mediated allergic reactions, see amoxicillin rash). Anaphylaxis will occur in approximately 0.01% of patients. There is perhaps a 5%-10% cross-sensitivity between penicillin-derivatives, cephalosporins, and carbapenems; but this figure has been challenged by various investigators. Nevertheless, the risk of cross-reactivity is sufficient to warrant the contraindication of all β-lactam antibiotics in patients with a history of severe allergic reactions (urticaria, anaphylaxis, interstitial nephritis) to any β-lactam antibiotic. A Jarisch–Herxheimer reaction may occur after initial treatment of a spirochetal
Broad-spectrum antibiotic that lists the susceptibility of community-acquired and hospital-acquired bacteria is helpful in guiding empiric therapy. Many professional organizations (for example, the Infectious Disease Society of America) publish guidelines for empiric antibiotic therapy, as do hospitals, with their choices tailored for their specific resistance patterns. Many of these guidelines also offer guidance on antibiotic dose and duration of therapy. Once a specific species has been identified and its susceptibilities determined, antibiotics can be "narrowed" to a medication which targets a more specific range of bacteria. If no specific species are identified, patients may continue on the empiric regimen. There are an