Honesty (drug)

From Tardis Wiki, the free Doctor Who reference

Honesty was a mood drug. Cheen wears an Honesty patch on her neck and displays it to Martha Jones to show she is being truthful to her word that she and Milo will drop Martha off once they’ve reached their destination via the Fast Lane. Martha later angrily rips the patch from Cheen’s neck, as she thinks she shouldn’t be using it due to her pregnancy. (TV: Gridlock)