Doctor Robert's an odd song for me, because I don't dislike it, but lyrically, it just feels like a joke that doesn't quite land. If it's about an actual American doctor who was doling out amphetamines to the rich, I'd expect it to be more pointedly mocking; if it was about their LSD-dropping dentist, I'd expect some more personal feeling from it. In 1980, John said the song was about himself, which doesn't make a whole lot of sense to me. Of all the possible explanations, the one that seems to fit best is that it's about Robert Fraser, and the joke is that he's not a doctor at all, but he'd prescribe you something to make you feel better. I'm aware that the album came out in 1966 while the band was still touring, and this was about as overt as drug references could get at the time, but it still bugs me slightly that the lyrics don't really go anywhere. There's a doctor who's giving out drugs, and that's it, that's the whole story.
Of course not every song needs to be incredibly profound, and even an album this good can be allowed a little bit of filler. But if anyone does find meaning in the song, I'd be curious to hear other perspectives.