In the sequence of Gem absorbing the boils, Kathryn Hays was strapped to a board and kept absolutely still while make-up was applied. Stop-motion photography filmed the progression. The few moments of the scene took eight hours to film.
This episode contains another one of McCoy's famous, "I'm a doctor, not a..." quotes. In this episode it is "coal miner".
The Empath was written by Joyce Muskat, one of only four fans who were able to sell scripts to the original series, the others being David Gerrold, Judy Burns, and Jean Lisette Aroeste.
After McCoy is tortured, his tattered uniform shirt is an older velour shirt, rather than the new polyester double knits that were used in the 3rd season.