Does anybody know what types of common medications can cause leukopenia (decrease in white blood cells)?

Answer 1

There are quite a few medications that can cause leukopenia:
benzthiazide
thorazine
cefadyl
ormazine
hydrex
Also, almost all cancer treatment drugs can cause leukopenia.
Those are the ones I can think of off the top of my head. Here is a list of many more:
http://www.wrongdiagnosis.com/symptoms/leukopenia/side-effects.htm

Answer 2

Almost all cancer treatments and immune modulators can cause this.