It is rumored that the song "Mellow Yellow" is about smoking dried banana skins, which was believed to be a hallucinogenic drug in the 1960s, but this rumor has since been debunked. According to Donovan's notes accompanying the album "Donovan's Greatest Hits" the rumor that one could get high from smoking dried banana skins was started by Country Joe McDonald in 1966, and happened to occur three weeks before "Mellow Yellow" was released as a single. Thus, the misconception of the song's meaning soon followed its release.
The song's title actually refers to the fact that Donovan had suffered from liver disease in the winter of 1966 and had become severely jaundiced.