Propecia (finasteride) is used to treat male pattern baldness (androgenic alopecia) in men. [It is also used in combination with doxazosin in the treatment of benign prostatic hyperplasia (BPH).] Finasteride is an orally active testosterone Type II 5-alpha reductase inhibitor, inhibiting the enzyme that converts testosterone to dihydrotestosterone (DHT). Topically (on the scalp itself) it is of some effect in minimizing further hair loss. In combination with Nizoral and spironolactone (which smells awful, by the way) it can actually reverse loss moderately.
Oral use though will reduce DHT levels systemically, which may adversely affect training and sex drive.
For use in cycles with testosterone, I don’t think it is bad at all. One simply doesn’t want less DHT than normal. As the amount of testosterone in the system increases, the amount of finasteride needed to keep levels down to normal increases. I consider 5 mg/day reasonable at the gram per week level, and proportionally less at lower dosages of testosterone.
I personally wouldn’t use more than the recommended pharmaceutical dosage, or 5 mg/day. More might not be safe and might at some point cause DHT levels to become abnormally low.
Finasteride is the chemical name of active ingredient in Proscar and Propecia. Proscar and Propecia are registered trademarks of Merck & Co. Inc. in the United States and/or other countries.