Endorphin

Endorphins were involved in the "chemical war" described by the Tenth Doctor, which was going on in Donna Noble's head as she walked down the aisle for her Christmas Eve 2007 wedding. He briefly hypothesised that the adrenaline, acetylcholine and endorphins had her particles reaching "boiling point", perhaps leading to her sudden appearance in the TARDIS. (TV: The Runaway Bride)

Alien symbiotes called hitchhikers could be considered beneficial to the human body, as they released endorphins into the blood stream. Ianto Jones noted that Mr Williams must therefore have died a happy man. (TV: Children of Earth: Day One)