prob won't prevent it from working, but also won't do much good at preventing the 1-ad/1test induced hair loss (te) or shedding. propecia (finasteride) only effctive at preventing the test to dht conversion via 5ar. 1test (and stuff like winny, proviron, etc) do not readily convert to dht via 5ar, and can cause hair loss due to inherent androgenicity. for these types of dht derivative supplement/hormones need topical spironolactone 5% (see minoxidil . c o m) which will block the androgen receptors when applied to scalp.
also, don't take propecia with 19-nor (nandrolones deca etc) prohormones. will actually make it worse on hair (too tired to explain, but do search here or on bodybuilding . c o m)