Finasteride (propecia, proscar) !? Pharmacologists, doctors, biochemists, balding guys, etc. HELP!?

I'm taking finasteride currently to combat the onsetting effects of androgenetic alopecia which has been bestowed upon my family like some kind curse out of hell. Anyways, I was wondering if it matters at all if one takes propecia or proscar. I USED to take propecia 1 mg, specifically marketed for male-pattern hairloss but NOW I am taking proscar 5 mg, which is marketed for preventing the development of prostatic cancer. I use a pill splitter to cut one 5 mg pill into two 2.5 mg halves. My actual concern resides in the cutting of the pill. I use one 2.5 mg half each day, leaving the other half in the blister pack, which is obviously now ruptured exposing the ingredients of the pill to the atmosphere for around 24 hours. Is this OK!?!? I mean, is there any denaturing of the ingredients or diminishing of their effects occuring when exposing this cut pill to oxygenation? Is it still effective? It's cheaper to cut proscar and cut it up than it is to buy propecia and swallow it whole. Help.