Will a judge look at a past history drug abuse with prescription medicine in a child custody case?

Almost certainly, yes, but just because the court considers such evidence does not mean that the court will find it a compelling reason to award you child custody. If the parent's current circumstances are stable and sober, the past usually doesn't weigh all that heavily on the here and now decisions before the court (some judges are exceptions, but that's my general view).

Probably - the best predictor of future behavior is past behavior.

I believe a Judge would look at anything that would affect the best interests of the child.