After a long trip (1836-1840) in the Orient, he discovered the effect of marijuana. He studied it in order to understand the relation between madness and dreams, which are similar deliriums, according to Moreau.
He was the first doctor to publish a work about a drug and its effect on the central nervous system. His first publication was the 1845 "Du Hachisch et de l'aliénation mentale" ("Hashish and insanity").
He is the creator of the Club des Hashischins.