The illegal Venus drug.
The Venus drug is a chemical compound described to give the user "more" of whatever they already have. Women become more beautiful and alluring, men more aggressive and muscular.
As of 2266, the drug was illegal in Federation space. Harcourt Mudd gave it to three women whom he was ferrying to Ophiucus III, as wives for settlers there. After the USS Enterprise intecepted Mudd's ship, these women ended up on Rigel XII, where the deception was uncovered. There, Eve McHuron digested a placebo and became as beautiful as she had been after taking the drug, necessitating the question of how effective the Venus drug actually was, and how much of a person's beauty is due to poise and confidence. (TOS: "Mudd's Women")