Tricillin PDF is broad-spectrum abti-viral that is effective against a great number of illnesses that afflict the assorted humanoid species of the Star Trek Universe. While little has been revealed about it, it seems to be a commonly-used drug across the Federation, and its name implies that it may be an antibiotic.
The most prominent use of Tricillin PDF was during the so-called ‘Double Helix’ incidents, where the drug proved effective at slowing the effects of some strains of the ‘Double Helix”, a series of weaponized viruses. For one particular strain, it halted the progress of the disease entirely, but it was not a cure. Extensive use of Tricillin PDF was successful is prolonging the lives of many of the infected until cures could be found, however.