The tears of Elasian women, which contain a biochemical agent that makes any man whose skin they touch fall helplessly and permanently in love with the woman who shed them.
Elasian tears act as a powerful love potion. According to the Troyian ambassador Petri, a man touched by them is bound to the woman for life, and the men of Elas have been conquered this way for generations.
In the original series episode Elaan of Troyius the Enterprise carries Elaan, the Dohlman of Elas, to an arranged marriage meant to end the war between her world and Troyius. Furious at being taught manners by Captain Kirk, she weeps, and he brushes a tear from her cheek. He is at once infatuated, to the alarm of Spock and McCoy, while a Klingon warship closes in.
McCoy works on an antidote in sickbay, but by the time he has one Kirk no longer needs it. Spock observes that the captain's devotion to the Enterprise proved the stronger attachment, and Kirk delivers Elaan to Troyius.