Source of some flashbacks

LSD

This psychedelic compound, first synthesized in 1938, is known for altering perception and was famously studied in mid-20th century psychological research.

"Source of some flashbacks" hints at LSD because the drug is famously associated with vivid hallucinations and intense sensory experiences that can resurface later as flashbacks. The clue plays on the dual meaning of "flashbacks"—both as sudden memories and as a well-known side effect of LSD use. It’s a straightforward reference to the drug’s psychedelic properties and its cultural reputation.