As might be expected from the different conditions under which plants grow, the different methods of collecting, drying, and preserving drugs, the effects of age on the drug, etc., crude drugs vary in strength. On this account the use of active constituents by themselves has much to commend it, e. g., quinine in preference to cinchona, strychnine in preference to nux vomica, resin of podophyllum in preference to podophyllum. These substances tend also to be more readily absorbed when thus separated from the extractive matter of the crude drug. But in many instances it is impossible or too expensive to isolate the active ingredients in pure form, or there is a preference for the combinations or mixtures as they occur in nature, so pharmaceutic preparations, and even the powdered crude drugs, are much prescribed, even though their active principles are available.

This being the case, it is a matter of great importance that some of the more potent of these drugs and preparations are standardized by the Pharmacopoeia to contain a definite percentage of the active ingredients. For instance, when assayed by the process specified in the Pharmacopoeia, nux vomica must yield not less than 2.5 per cent. of alkaloid; jalap, not less than 8 per cent. of resin; the tincture of opium, 1.2 to 1.25 per cent. of morphine. These are known as assayed drugs or preparations.

An assay process is a process by which the strength of a substance or preparation is determined. There are three kinds of assay processes for drug preparations, viz., volumetric, gravimetric, and biologic or physiologic. The last-named type of assay has been devised for some of the drugs whose active principles are not readily isolated. For digitalis, for example, one assay process ascertains the amount of digitalis necessary to bring into systolic standstill the heart of a frog of definite weight and of a certain species and sex.