While The Beatles were no strangers to drugs prior to 1965, their introduction to LSD caused a major shift in their music, personalities and public perception. LSD (lysergic acid diethylamide) is a psychedelic drug that alters the thinking process, closed- and open-eye visuals, sense of time and spiritual experiences. The drug played a key role in the 1960s counterculture. LSD was a key influence in the changes in The Beatles' music between 1965 and 1968. The drug played a pivotal role in the group's studio experimentation for Revolver and Sgt. Pepper's Lonely Hearts Club Band.
LSD was introduced to John Lennon and George Harrison by a dentist at a dinner party who slipped a laced sugar cubes in their coffee. After that experience according to George Harrison, he and Lennon had decided that the other Beatles should experience LSD, which they had previously taken between March and July 1965. George was quoted stating "John and I had decided that Paul and Ringo had to have acid, because we couldn't relate to them anymore. Not just on the one level - we couldn't relate to them on any level, because acid had changed us so much. It was such a mammoth experience that it was unexplainable: it was something that had to be experienced, because you could spend the rest of your life trying to explain what it made you feel and think. It was all too important to John and me" .
LSD had a profound effect on The Beatles' songwriting and recording. The first-released song to mention it was Day Tripper, but over time its influence resulted in less explicit and more abstract references to acid. In the song it can be seen as John poking fun at Paul for not taking LSD. When The Beatles were usually confronted by people to see if their lyrics were in fact drug references they would always joke around. "Day Trippers are people who go on a day trip, right? Usually on a ferryboat or something.