A group of 386 patients was studied for the possible effects of metronidazole on the white blood cell (WBC), relative percentage of polymorphonuclear leukocytes to lymphocytes, and absolute polymorphonuclear cell counts. The only change directly attributable to metronidazole is the occurrence of neutropenia in ten of the cases. This neutropenia in all instances was transient. In the event of a failure of cure or of a reinfection, WBC and differential counts should be made before instituting another course of therapy. It would seem logical (1) to do a leukocyte and differential count before and after each metronidazole treatment in those persons showing some evidence of health deficiency, (2) to treat only proven cases of trichomoniasis, and (3) to await further studies before giving metronidazole to pregnant patients.
Lefebvre Y, Hesseltine HC. The Peripheral White Blood Cells and Metronidazole. JAMA. 1965;194(1):15–18. doi:10.1001/jama.1965.03090140023005