So I don't have this book, but I found this in the Google books preview.
Apparently he was doing "drugs" (not specified) at least until the early 90's?
From 'George Harrison: Behind The Locked Door' by Graeme Thomson, page... something. Google didn't say....
"During a rather sad and slightly drunken call from Sydney in the summer of 1985 Harrison told Michael Palin that he had given up all drugs and kicked (for now, in any case) cigarettes, but he had periods of indulgence much later than that. He told another old friend, Al Aronowitz, in the early Nineties that his excesses "got me to the point where I said, 'Jesus! I gotta do something here!'" The solution, he said, was meditation. "I had forgotten totally that that's what it was all about - to release the stress out of your system. And I got back into that. I do a double-dose now and it's like, say, whereas an alcoholic can't go through a day without going to AA or doing some kind of a program like that, for me it's th emeditation program. In order to keep myself focused and keep the buoyancy, the energy, and also to realise that all this stuff that's going on is just bulls***." "