Some drugs can cause a fall in the leukocyte count, especially neutrophils. Chemotherapy drugs are the most that can cause leukopenia, and it is dose-related. In other words, the higher the dose, the greater the effect on the number of cells. Other drugs can also affect leukocyte count, including antiepileptic drugs as lamotrigine, antibiotics as penicillin, vancomycin and trimethoprim-sulfamozxazole, antimalarials such as hydroxychloroquine, and even some thyroid medications as methimazole. It is worth noting, however, that the risk of having leukopenia due to non-chemotherapy drugs is minimal and much less compared to chemotherapy.