My wife was at the doctor for salmonella poisoning and is allergic to penicillin. The doctor prescribed generic Bactrim and she has been taking this medication for two days. She now has itchy hives-looking patches on her legs and lower back. I researched and saw that hives are a side effect that happens rarely. Is there a common chemical compound between the penicillin family and the Bactrim family. She needs the antibiotics due to her sickness but I am concerned about her continuing to use them because of the outbreak. Is there a similarity and should she continue use and count herself as one of the rare patients who receive hives from the sulfa class of antibiotics