The hippies and other counterculture movements weren’t the only groups to experiment with mind-altering substances. Starting in 1953, the Central Intelligence Agency conducted research on psychedelic drugs as part of a top-secret behavior modification program codenamed MK-ULTRA. This bizarre project saw the CIA undertake an extensive—and ethically dubious—series of psychological experiments involving hypnosis, shock therapy, interrogation and hallucinogens like LSD.

MK-ULTRA’s LSD experiments were initially done on willing volunteers—novelist Ken Kesey was a famous participant—but they later took a more sinister turn. The CIA believed that a tasteless, odorless compound like LSD might be used as a truth serum or psychological weapon in the cloak-and-dagger operations of the Cold War, and they even concocted schemes to dose Cuban leader Fidel Castro. To observe the effects of the drug on unwitting subjects, they secretly administered LSD to hundreds of mental patients, prisoners, foreign nationals and private American citizens without their consent. In one particularly audacious eight-year program called “Operation Midnight Climax,” the agency set up a string of brothels in San Francisco. Prostitutes would dose their unsuspecting clients, and CIA agents would then monitor the men’s behavior from behind two-way mirrors. These government acid tests would continue through most of the 1950s and early 1960s, but the mind-bending effects of LSD were ultimately deemed too unpredictable and unwieldy for use in the field. Many of the test subjects experienced adverse psychological reactions, and at least one—a biochemist named Frank Olson—died under mysterious circumstances only days after ingesting the drug.

Project MK-ULTRA was scaled back extensively and then terminated in the late 1960s, and the CIA destroyed most of the program’s records in 1973. A cache of surviving documents relating to the agency’s “mind control” program was eventually discovered and revealed to the public in 1977, but the full extent of the government’s psychedelic experimentation remains uncertain.