A powerful stimulant that Leonard McCoy injects into Spock on Gamma Trianguli VI after the Vulcan is struck by poisonous thorns, saving his life at the cost of an upset stomach.
Masiform D is a 23rd-century injectable stimulant seen in the Original Series episode The Apple. When the landing party explores Gamma Trianguli VI, a pod plant fires a cluster of poisonous thorns at Captain Kirk; Spock pushes him aside and takes them in the chest.
The same thorns have already killed a crewman outright, but McCoy treats Spock on the spot with a hypospray of Masiform D and the Vulcan recovers within moments, thanks in part to his hardier physiology.
Spock's only complaint afterwards is that the drug has left him feeling sick to his stomach, which McCoy takes as a welcome sign that his patient is back to normal. The compound is not mentioned again in the series.