Neutropenia associated with mezlocillin and piperacillin.

Neutropenia associated with beta-lactam antibiotics has been widely reported since the first case was described in 1946. Cross-reactivity between beta-lactam antibiotics for this phenomenon rarely has been reported. This case describes the development of neutropenia after a course of mezlocillin, resolution upon discontinuation, and recurrence promptly after initiation of piperacillin, with resolution subsequent to discontinuation. Clinical practitioners should be aware that this adverse drug reaction has been associated with the newer beta-lactam antibiotics and that cross-reactivity may occur between these antibiotics.