Elasian tears
Elasian tears, tears secreted by Elasian females, contained a biochemical compound transmittable to Elasian (and Human) males through crying. Upon skin contact, men became romantically infatuated and somewhat enthralled to the woman who infected them. 

Men of Elas tried for centuries to dispel the tears' "mystical powers" without success. Troyian Ambassador Petri said of the phenomenon, "A man whose flesh is once touched by the tears of a woman of Elas has his heart enslaved forever."

In 2268, Captain James T. Kirk was infected by the tears of Elaan, the Dohlman of Elas. Although he was visibly affected and drawn to Elaan, he managed to overcome the tears' more compelling effects in a short time – just as Dr. Leonard McCoy believed a cure was within reach as they acted like an infection. First Officer Spock speculated that the  was all the antidote Kirk needed for Elasian tears. ()

See also 
 Love crystal
 Love potion
Drugs
Biochemical compounds