Which medication is contraindicated in patients with a history of anaphylaxis to penicillin?

The contraindication of cephalosporins in patients with a history of anaphylaxis to penicillin is primarily due to the structural similarities between these two classes of antibiotics. Cephalosporins, like penicillin, are beta-lactam antibiotics, and this shared chemical structure can trigger allergic reactions in sensitive individuals.

Patients who have experienced anaphylaxis to penicillin are at a heightened risk of having similar or cross-reactive reactions to cephalosporins, particularly the first-generation cephalosporins, which have a higher chance of cross-reactivity. Therefore, it is standard practice to avoid administering cephalosporins to patients with a significant penicillin allergy history, especially when there are severe allergy manifestations like anaphylaxis.

Amoxicillin and other penicillin derivatives pose the same risk for these patients, so they are also contraindicated. Doxycycline, being a tetracycline antibiotic, does not share this beta-lactam structure and does not typically cross-react with penicillin allergies, making it generally safer for these patients.