The original version of this was released by a blues group called Dr. Feelgood and The Interns in 1962. It was written by Roy Lee Johnson, who was a guitarist with The Interns. This original version the song was a B-side to the single "Dr. Feelgood" (no relation to the Motley Crue album and song of the same name
), by the aforementioned group. As if that didn't muddy the waters enough, there's also a TV series Moonlight
with an episode titled "Dr. Feelgood," which appears to be a complete coincidence.
The song is best known for its Beatles cover, but even among hardcore Beatles fans, this song doesn't have as much enthusiastic support. It was part of their standard stage set for many years, in part because it gave John a chance to show off his blistering vocals on stage.