I think it is because the Histamine 2 receptor Antagonists inhibit both basal secretion of gastric acid and the secretion stimulated by histamine, acetylcholine, and gastrin. For example, Cimetidine (Tagamet) is the prototype of this class. It inhibits action of histamine at H2 receptors of the stomach, decreasing the amount of acidity, and pepsin content of gastric juices.
