Abnormally decreased white blood cell count following use of certain medications. Most commonly involves neutropenia but may also affect other leukocyte cell lines. Among the drugs that can cause leukopenia are antithyroid drugs, chemotherapy agents, antimetabolites, phenothiazines, antihypertensives, antihistamines, monoclonal antibody drugs, antivirals, anticonvulsants, antipsychotics, anti-inflammatories, antibiotics, and alkylating agents.