Think about music, and the number of truly great musicians who succumbed to one or both. I often wonder if the damage is an effect of fame, or if fame is an effect of the addictive behavior (or in that vein maybe the public is somewhat perversely attracted to people falling deeply into drugs or alcohol).
I honestly believe that more often than not, they are self medicating. Some people have problems before fame, some after the fame hits. The human mind can be a very fragile thing sometimes, and a good percentage of people (famous or not) look to ease the discomfort they are experiencing. They turn to alcohol and or other drugs to dull the feelings they have.
Life has many twists, turns, and different paths that can be chosen. Not all paths lead to a good outcome.