Specifically, posaconazole’s interaction with cytochrome P450 enzymes and P-glycoprotein complicates its use in patients requiring multiple concomitant transplant-related medications or newer targeted therapies for leukemias such as isocitrate dehydrogenase inhibitors.6 In addition to these drug-drug interactions, common toxicities of posaconazole include gastrointestinal effects (18%), headache (5% to 17%), fever (12%), dry mouth (9%), neutropenia (7%), musculoskeletal pain (7%), and liver toxicity (5%).11,12. The gene discussed is ABCB1; the disease is Decreased total neutrophil count.