Lactucarium

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 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.

Although the effect is not that comparable, with opium being so much more potent, it still is effective in people who have no opioid tolerance.