My opinion is that Elvis didn't get involved with pills just for pure
FUN, at least at the beginning.
He started with sleeping pills when he was in the army because he always had problems of insomnia; he also took others kind of pills (given to the GI) to stay awake under the snow, during the night.
If Elvis had wanted to take pills just for fun he would have taken them from the beginning, when he started to tour in the fifties.
When he started he felt that pills could really help him, without any idea to depend on them.
With the years there were others resons: life style, hard work and, at the end a bit of dissatisfaction and depression....it became something normal for him to face his business and life in general in that way.
I think he realized at the end what he was doing to himself but he just couldn't stop it, maybe because he hadn't enough will power.
It's really sad 'cause without those pills he would have lived more but I think it's really hard for us to understand completely what it meant to be "Elvis Presley"