While up to 10% of patients may think they have an allergy to beta-lactam antibiotics, fewer than 1% have a true IgE-mediated hypersensitivity, with concomitant risk of anaphylaxis. However, even if a true allergy is confirmed, this does not mean that the patient cannot receive any beta-lactam antibiotics. Whether or not there is likely to be cross-reactivity between the antibiotic to which the patient is allergic and another beta-lactam antibiotic depends on whether their structures share similar R side chains, as explained in this article. A handy can be kept on your mobile phone delineating which antibiotics cross react.