Some people are just built different.

I know atleast one old guy who tells stories about the shit he's done, who functions perfectly in society, self-employed, and reliable as the day is long.

But he's damn near done all the drugs, barring the nastier shit coming out nowadays. Boggles my mind.

Pretty much my first thought. The fact he hadn't died yet is remarkable.

He was an old, British rocker. Those guys just don't die.

By making sure you have a healthy balance of all the drugs.