MJ was under great stress for a long time now.
Ever since those Molestation charges started popping up like zits, he lost a great deal of money. And you know what? Im pretty sure it was just scam to get money off of him. I mean, it was easy to abuse of the fact that MJ never had a childhood. MJ as a 40 something adult, was living his childhood he never had. but certain things just cannot be done anymore, and they abused on that. They abused on how he liked kids because inside, he was one he never got to grow right. he went straight into being a superstar adult. and now, that he had all the money and fame in the world, he was ready to live his childhood.
Those things drained him ALOT. and not just physically and emotionally, but financially too. After all that, He really couldn't do it anymore. he was crushed I suppose. So he took all these things to help keep him in tip top shape. Painkillers to ignore the pain, this for that, and that for this... and certain medicines should not be mixed... Certain medicines should be taken in certain doses... But he needed it, so his physician said, sure why not. and this is what happened.