Adverse drug reactions and allergic responses to medications and vaccines are no longer rare side effects—they’re a growing medical challenge. In this episode, we sit down with Dr. Mariana Castells, Professor of Medicine at Harvard Medical School, Director of the Mastocytosis Center of Excellence, and Director of the Drug Hypersensitivity and Desensitization Center at Brigham and Women’s Hospital.We explore:Why newer medications and targeted therapies carry higher risks of severe reactionsThe difference between drug intolerance, adverse reactions, and true allergiesThe dangers of mislabeling penicillin allergy—and how it affects treatment choicesCutting-edge diagnostic tools like basophil and mast cell activation testingThe role of mast cells, novel receptors, and genetics in multiple drug allergiesStrategies for safer prescribing, testing, and desensitizationWhether you’re a clinician, researcher, or patient who’s experienced a “bad reaction” to medication, this conversation offers clarity on what we know today—and where the science of drug allergy is heading.