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We do not completely disagree with Dr Fellner's letter. All of our patients were receiving multiple-drug therapy, and we thought it was unsafe to give them challenge doses of any of the medications singly or in combination. We considered allopurinol as the prime offender alone or in combination with the other drugs because it was a common denominator in our three cases. Although allopurinol apparently increases susceptibility to ampicillin-induced rashes according to the Boston Collaborative Drug Survey, we are not cognizant that the survey showed that this combination causes classic toxic epidermal necrolysis.
Ellman M, Fretzin DF. Allopurinol-Caused Toxic Epidermal Necrolysis-Reply. Arch Dermatol. 1976;112(9):1327. doi:10.1001/archderm.1976.01630330081035