It is natural to assume that pot would serve as a substitute (higher use would decrease heavy drinking), but it is equally likely that it would be a complement (higher use would increase heavy drinking).
And two, how come someone as inexperienced as Baroness (Catherine) Ashton was chosen, rather than a heavy hitter like Lord (Peter) Mandelson, who was actively campaigning for the post in the last days before the EU summit on November 19th.