LSD

LSD (lysergic acid diethylamide) is a psychedelic drug used by The Beatles in the mid to late 1960s.

While LSD influenced the group's music, they rarely used it in-studio.

LSD was introduced to John and George by a dentist at a dinner party who slipped it into their drinks. While John, George and Ringo were the first to try it, Paul was the first Beatle to speak to the press openly about his use of the drug.

The Beatles speak about LSD

George: The first time I had acid, a light bulb went on in my head and I began to have realizations which were not simply, “I think I’ll do this,” or “I think that must be because of that.” The question and answer disappeared into each other. An illumination goes on inside: in ten minutes I lived a thousand years. My brain and my consciousness and my awareness were pushed so far out that the only way I could begin to describe it is like an astronaut on the moon, or in his space ship, looking back at the Earth. I was looking back to the Earth from my awareness.

Paul: Pot and LSD were the two other major influences. Instead of getting totally out of it and falling over, as we would have done on Scotch, we’d end up talking very seriously and having a good time till three in the morning.

John: We must always remember to thank the CIA and the army for LSD, by the way. Everything is the opposite of what it is, isn’t it? They brought out LSD to control people, and what they did was give us freedom.

George: If I had half a chance, I’d put acid in the Government’s tea.

Ringo: I think LSD changes everybody. It certainly makes you look at things differently. It makes you look at yourself and your feelings and emotions. And it brought me closer to nature, in a way – the force of nature and its beauty. You realize it’s not just a tree; it’s a living thing. My outlook certainly changed – and you dress differently, too!

Source: http://www.musicbyday.com/the-beatles-on-marijuana-lsd-in-their-own-words/585/