Inspired by a "dream journal" written by Hiroko Nishikawa, LSD is basically a simulation-based game themed around dreams and nightmares. The gameplay is simple, walking around various dreamscapes and occasionally discovering surreal visuals. Coming into contact with any object or wall in the dream can result in warping to another part of a dream, which can go on for very long (up to 10 minutes), or can be cut short by falling off cliffs or through floors. After the dream ends, it is displayed on a graph showing whether it is considered Upper, Downer, Static or Dynamic.
Gameplay takes place over various days, and after a long time passes, the counter resets, though the game can still be played. Textures can be completely randomized, and upon starting a new dream, you can not have the same one twice in a row, though dreams can be replayed using Flashback mode (unlocked after 20 days have passed), which is an option to revisit the same areas and allow you to choose different paths, though only for 3 to 5 minutes.
There is no goal in the game, except to emulate dreams, ranging from colorful and happy, to dark and unsettling. Occasionally, dreams may be invaded by an apparition called the Grey Man, who can appear out of nowhere and remove your ability to remember a dream in Flashback mode.