I remember reading excerpts from, I think, the Goldman book on John Lennon where Goldman talked about Yoko scoring heroin from a guy who would deliver it every day to her by the gram wrapped in foil. The delivery guy would go by bicycle from the dealer to the Dakota, ring up Yoko, and she'd let the guy in to her 1st floor office where she's make him wait while she ducked into the bathroom and snorted some of it. Then while the dope delivery guy waited she'd vomit loudly in the toilet, come out and pay him. She asked the guy to 'never tell John' about her habit. I remember reading about that.
But, I don't ever recall reading anywhere that Paul had a heroin habit. Mention has been made here and there on the forums about Paul being addicted to smack, but I can't remember anyone claiming they know he did this awful drug other than those mentions on a few posts in one or two different forums.
When people make this kind of comment I wonder if they really know these things or if they are just repeating verbatim something they heard in passing. The Goldman mention of Yoko and her habit is most likely true, but I couldn't swear by it. Who knows ? I just thought I'd bring this up because it makes you wonder.