Addiction, mostly heroin/morphine addiction.
Hey dad, you been smacking da H again, quit it bitch, you have a monkey on your back
But the morphine eased the pain,
And the grass grew round his brain,
And gave him all the confidence he lacked,
With a Purple Heart and a monkey on his back.
There's a hole in daddy's arm where all the money goes,
-John Prine - 'Sam Stone' a song about a vietnam vet returning to the US with a monkey on his back.