Beard suppressants, also known as beard repressors, were a method of preventing the growth of beards as opposed to the more traditional method of shaving. While effective, it required continual application to keep a beard from growing.
Doctor Leonard McCoy normally used a "beard repressor" to keep his beard from growing. In an alternate timeline in which James T. Kirk had been killed, Doctor McCoy did not keep up with the reapplication schedule, and the suppressant wore off. After noticing the beginnings of a beard he shaved and reapplied the beard suppressant. (TOS novel: The Entropy Effect)
Following the Enterprise's five year mission, Kirk and Kevin Riley were transferred to Starfleet Headquarters. When Kirk noticed that the latter was growing a beard, he asked him if he had run out of "beard suppressor". (TOS novel: The Lost Years)