Denny McClain, the last pitcher to win 30 games in a season (31-6 in '68) later became a compulsive gambler and spent time in prison. However, he had another addiction which was legal (he even got paid for it for a short time !)
What was it ?

wild guess
cigarettes?

Sorry, good guess, gt - this particular addiction is borne by many, but not to the degree that McClain took it - I know of no long term terrible side effects !
I am affected to a degree, myself...
I will post the answer later on this afternoon (15 hrs. ahead, gt ?) Probably will be by 08:00 Monday your time !

This question was a tough one and I only know it 'cause I lived there shortly after that.. Denny McClain was hooked on PEPSI ! He was drinking 18-20 16 ounce bottles per day !!! Pepsi was paying him for a while to promote their product, but his manager started fining McClain every time he had a Pepsi in his hand while in uniform, so it ended soon after..