I believe that once finasteride is digested and enters the bloodstream and does what it's supposed to do, it's out of your system in about 4-5 hours. However, DHT conversion isn't something that happens once a day, the way a pill is often taken. It's a constant process, so suppression of 5AR is probably better off being done daily in smaller doses than taken twice a week in large doses. That's pretty much the basic concept that my derm told me, but I could be wrong. That appointment was in April and I was so thoroughly excited to be getting a script for finasteride that I didn't even pay that close attention to what was being said.