Back before TMZ was filming the every move of every celebrity, there were many who were successful and on drugs. Robert Downey Jr., for one. Elvis was popping pills throughout the 60s. Drew Barrymore has been sober for a long time now but did a lot of drugs. Johnny Cash had well documented problems with drugs. Keith Richards was a heroin addict and the Rolling Stones kept making successful records. Even Daniel Radcliffe has recently said he was drunk for some of the latter Harry Potter films.
These people have handlers who help them manage their stardom and their drug use, often enabling. A decade or so ago you would never know these people were in trouble because their publicity people would never let it get out. Then the gossip technology (the internet) became more invasive, and it's harder for them to keep their secrets.
You can't be so naive as to think every successful singer or actor is going home, working out, drinking vitamin water, and getting 8 hours of sleep.
Daisyx3