Isn't there evidence that John continued to use heroin into the 70s? I forget which John book it was in, but John arranged some press meeting-- perhaps John Sinclair, perhaps an album announcement, I honestly don't remember. And the press didn't take pictures or report the story or John's "message", because he was so obviously wasted. Is this just Beatle rumor?
I think Harihead is correct. I know Goldman deals with John's ongoing cocaine and heroin use in the 70s (and portrays his "house-husband" phase more as a strung-out phase). Not sure what Fred Seaman or May Pang or others have to say about John and heroin.
Actually, Spanish Tony Sanchez has a very simple, but compelling explanation for heroin addiction. He explains that Keith, himself and others, got hooked on heroin to come down from cocaine (the pleasure of oblivion after the pleasure of hyper-awareness).
John certainly had a junkie look to him in those Annie Leibowitz pictures taken the day he died--although this skeletal skinniness may have been a result of his "macrobiotic diet" or anorexia.
As Neil Young says:
"Fourteen junkies too weak to work,
one sells diamonds for what they're worth
down on Pain Street disappointment lurks"
Sadly, I'd say, Mr. Lennon took up residence on Pain St.