A substance (usually a drug) that causes someone to see, hear, or perceive things that aren't really there.
From Latin hallucinari (to hallucinate) combined with the Greek suffix -gen meaning 'something that produces or causes.' The word was created in the 20th century for psychoactive drugs.
The term hallucinogen was actually coined to be more scientific and less judgmental than older names—it's a fascinating example of how language evolves to reflect changing attitudes about drugs, from 'demon possession' to medical terminology.
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