Britney has ALWAYS had really bad skin. If the skin is the clue of meth use in her case then Kevin was well aware of it before he married her and had kids with her.
There were many a picture of her with white cream on her zits before they got married. She is just prone to shitty skin.
I really don't think she's on meth. You would see signs of a runny and/or red nose with the amount of use they are saying she does. Unless of course she's doing it as cat on a hot tin roof, which would be too much work for Brit.
The Britney Spears That You Have Never Seen Before Album