The prescription should be as brief and simple as possible. It should be explicit and clearly written. It may be expressed either in Latin or in English. The manner in which the medicine is to be used should be specified. Important instructions as to the rule, systematic regulations or diet of the patient are sometimes necessary.

Prescriptions usually contain two or more of the following four representative constituents: (1) The basis or active ingredients. The practice of conjoining several active medicines has wisely been abandoned. Occasionally, however, it may be advantageous to give together two medicines producing their effects in somewhat different ways. Thus, spasms of the bowels are more often effectually controlled by the conjunction of a stimulant like ether and an anodyne like opium than by either given alone. Pain which is not alleviated by either morphine or atropine is sometimes abated by giving them together. (2) The adjuvant is introduced in order to increase, moderate or modify the action of the basis. Frequently its chief object is to insure solubility and ready absorption. (3) A corrective is occasionally required to temper the effects of the basis. Thus a small dose of opium is prescribed with oil or other laxative in cases of diarrhoea; ginger is generally added to the aloetic mass to prevent its griping. (4) The vehicle generally consists of some comparatively inert substance, added to facilitate administration, such as the treacle, linseed meal or licorice powder used as an excipient for boluses and pills, the benzoated lard or vaseline used for making ointments, and the water given in drenches.

Example:

In the above prescription aloes is the basis; calomel as an adjuvant, ginger as a corrective, molasses as an excipient.

A prescription is composed of several parts, which may be considered as follows: