He is most famous for synthesizing and experiencing the effects of LSD from ergot-derived alkaloids; ergot is a fungal pathogen that grows on grain plants. He then identified psilocybin as the active psychoactive component of magic mushroom samples from Mexico.
When he turned his work to identifying the active component in Morning Glory plants, he presented his work showing that he'd discovered LSA, another ergot alkaloid. Other researchers accused him of having contaminated samples, because he'd found in plants compounds which were known only from the fungi kingdom. Hoffman's work was vindicated, in a sense, when the relationship of endosybiotic fungi (cryptic fungi which spend the majority of their lifecycle inside a plant) was later elucidated.