amboss doesnt mention bullae at all in EM article https://next.amboss.com/us/article/Tk06MT?q=erythema+multiforme#Z2f07743e6ff2abf96457612acfa6699e … also says “Stevens-Johnson syndrome: In comparison to EM, skin involvement in SJS is more severe and usually triggered by drugs. (Stevens-Johnson syndrome (SJS) is considered to be either a severe variant of EM major or its own distinct entity; no definitive conclusion has been reached yet.)”