Lactucarium is the milky fluid secreted by several species of lettuce, especially Lactuca virosa , usually from the base of the stems. It is known as lettuce opium because of its putative sedative and analgesic properties. It has also been reported to promote a mild sensation of euphoria. Because it is a latex, lactucarium physically resembles opium, in that it is excreted as a white fluid and can be reduced to a thick smokable solid.
Bremia lactucae is a plant pathogen. This microorganism causes a disease of lettuce (Lactuca sativa) denominated as downy mildew. Some other strains can be found on 36 genera of Asteraceae including Senecio and Sonchus . Experiments using sporangia from hosts do not infect lettuce and it is concluded that the fungus exists as a quantity of host-specific strains (formae speciales). Wild species, such as Lactuca serriola , or varieties of Lactuca can hold strains that infect lettuce, but these pathogens are not sufficiently common to seriously infect the plant.