It was spring 1965. Lennon and his wife, Cynthia, and Harrison and his wife, Pattie Boyd, were attending a dinner at the London home of dentist John Riley and his girlfriend, Cyndy Bury. Before the foursome left, Riley asked them to stay for coffee, then urged them to finish their cups. Shortly after, he told Lennon he had placed sugar cubes containing LSD in the coffee.

Beatles’ Acid Test: How LSD Opened the Door to ‘Revolver’ | Rolling Stone