Beard suppressants 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 suppressant 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)