Ain’t no “supposedly” about it. She’s sometimes visibly strung out during Jedi, and she’s got what I call the “coke voice”–quite raspy and constantly annoyed. Listen especially when she and Luke are talking on Endor. She landed in rehab in the mid-80’s and has now been reportedly straight for years.
There’s an anecdote in the commentary to the Empire DVD about how the night before shooting the arrival at Bespin Carrie, Harrison, Billy Dee Williams, etc. were partying with Eric Idle and the Pythons when, in Fisher’s words, “The Rolling Stones came over.” She says no one had slept when they shot the scene the next day and that their smiles were genuine, if you know what I mean.