This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
Drugs produce in the organism, when given in sufficiently large doses, certain disturbances or alterations of function, usually of a correspondingly definite character. The dosage required for this purpose is, as a rule, a fixed one within certain limits. It is the physiological dose - that is, a dose large enough to produce symptoms. Opium constipates the bowels, produces insensibility. For these purposes a recognized, fixed quantity is necessary, not less than one-half to one grain. This constitutes its physiological dose. Strychnia increases the reflex excitability of the spinal cord, in doses of one-twelfth to one-thirtieth of a grain. Digitalis slows the heart in ten minim doses.
Now this direct, absolute action of drugs, which is constant, can be made the basis of treatment of disease, wherever this is possible. Its advantages are immediate results and improvement of certain conditions opposed to this direct drug action. It is, therefore, palliative where indicated. This use of drugs is based upon the law of contraria contrariis opponeyida, when an opposite result is desired, or when it is intended to produce not an opposite, but an entirely different action, as, for instance, a purgative in a case of headache. The objections to this direct use of drug effects, by means of physiological dosage, are the limited field to which such action is applicable and the necessity for increasing dosage, and sooner or later opposite reactionary results that make further use of the drug useless. All physicians may make use of this direct, physiological action of certain drugs for certain conditions, but its usefulness is limited. Hahnemann himself clearly defines it as follows:
"I do not fail to recognize the great ability of palliatives. They are often not only quite sufficient in cases appearing suddenly and developing rapidly, but they have great advantages, indeed, where aid cannot be postponed for an hour, or even a minute. Here, and here alone, are palliatives of real use".
This use of drugs for their direct primary effects by means of a dosage sufficiently large and within certain limits, always definite and precise, has led to a classification of drugs, according to their physiological and some of their therapeutic actions, and not differing materially from that introduced by Dioscorides, the father of Materia Medica. Since his time drugs have been classified into three principal classes, evacuants, alteratives and specifics. The evacuants are again subdivided with respect to the various routes by which nature expels the morbid matters, such as purgatives, expectorants and diaphoretics. Alteratives comprise drugs which alter the course of morbid conditions, modifying the nutritive processes while promoting waste, and thus indirectly curing some chronic diseases; such are Mercury, Iodine and Arsenic. They increase metabolism.
Other classes are the antipyretics, emmenagogues, styptics, anthelmintics, astringents, etc., etc.
While such classification is very imperfect, and but a partial designation of the properties of drugs, for every drug may belong to several classes, and its special properties in any class are at best vague and uncertain, still, there is some advantage to those who want to avail themselves of the direct drug effects, of this drug classification, based on some of their more marked pathogenetic and therapeutic effects. But it is entirely useless for homoeopathic prescribing.