The stems of lettuce plants ooze a milky juice when cut which is similar in which is similar in taste and appearance to opium. The ancient Egyptians used it and it was allegedly listed in the Pharmacopeia of the United States of America until 1916.
It remained popular throughout the 19th century but nowadays, other than a brief stint in the mid-1970s, lettuce opium as a legal mind-altering drug by the hippie movement its largely unused except by the herbal industry.
For those of you fancying a little "trip" into the unknown the journal Science states that lettuce contains 2 to 10 parts of morphine per billion. Which means that with a usual therapeutic dose of morphine being 0.5 to 50 parts per thousand (a million times more), that you'd need a lot of lettuce!