I just started, not under the guidance of a derm but just prescribed from my doctor, he gave me instructions with 10 to 20mg per day and he'd relook at it after 3 months, if we had to go up to 40mg he'd monitor my liver values.
I thought I'd just start at 20mg straight away, but fuck that shit, within a week I had dry skin, face was dry, hands, lips, inside my nose, which was ok but the thing that got me was I had a flushed red face, it looked like I was sunburnt, and I got a few comments asking if I'd been out in the sun too long so I dropped back down to 10mg and its all cleared up, I'm going to try going back to 20mg in a month.
My advice is this is a powerful med, don't fuck about with it, take it low and slow, it hasn't even touched my shoulder acne yet but I know it's just a matter of time.