Tonics are medicinal agents which impart strength to the system, producing a gradual and permanent increase of nervous vigor, without preternatural excitement. They stimulate also in a secondary manner, by gradually increasing the force of the circulation, and restoring the digestive and secretory functions to a more healthy state, differing, however, from stimulants, in the more permanent character of their effects.

Certain agents of this class, when administered in large doses, act as antiperiodics in intermittent diseases. Benefit also results in the use of tonics by alternating them. They are divided into vegetable and mineral tonics, the former possessing a bitter extractive principle, and the latter uniting astringent with tonic properties, and, in the case of preparations of iron, increasing the red coloring matter of the blood. Such agents as gentian, quassia, calumba, wild cherry, serpentaria, cinchona and its alkaloid quinine, salix, pepsin, are examples of vegetable tonics; and the preparations of iron, preparations of copper, preparations of zinc, and such agents as subnitrate of bismuth, sulphuric, nitric, muriatic and oxalic acids, are examples of the mineral tonics.