Cross-Allergenicity of Cephalothin and Penicillin

To the Editor:—  The editorial in the Sept 8, 1969, issue of The Journal by Robert G. Petersdorf, MD, was appropriate and concise. The statement that caphalothin sodium can be substituted for penicillin when a patient gives the history of penicilgy implies that cross-allergenicity between cephalothin and penicillin does not occur. I know that Dr. Petersdorf is aware of the similarity of the chemical structure of these two antibiotics. Since severe systemic reactions have been reported to occur in patients treated with cephalothin when it was substituted due to penicillin allergy, we must use cephalothin with caution in patients who are allergic to penicillin. While it is true that the cross-allergenicity is weaker with cephalothin, it would be unfortunate to disregard this type of allergy.Dr. Petersdorf is certainly quite familiar with this cross-allergenicity between cephalothin and penicillin and I only bring this point up because the editorial reads as