A chemical agent that induces alterations in perception, thinking, and feeling which resemble those of the functional psychoses without producing the gross impairment of memory and orientation characteristic of the organic syndromes. Most hallucinogens are taken orally. Effects are noted within 20-30 minutes of ingestion (depending on the drug used) and include euphoria, visual hallucinations and altered perceptions. Rapid fluctuations between euphoria and dysphoria are common.