The dose of opium varies extremely with the purpose to he fulfilled, the idiosyncrasies and habits of the patient, and the modification of susceptibility produced by disease. For full anodyne and soporific effect, the medium dose is one grain. Less than half a grain will seldom produce these effects fully; and more than two grains should very rarely if ever be given, as a commencing dose, to an individual whose peculiar susceptibilities are unknown to the prescriber. For the pure stimulant effect, in persons quite unaccustomed to the use of the medicine, and in diseases in which the susceptibility is not impaired, the dose may vary from one-eighth to one-half a grain, which should be repeated, according to the quantity given, every two, four, or six hours, so as to sustain a steady impression. As a mere nervous stimulant, in the sense in which that expression is used in this work, to designate, namely, a certain class of medicines, having peculiar properties and applications, the dose is from one-twelfth to one-quarter of a grain. For this purpose, the liquid preparations are generally to be preferred. But it must be remembered, in regulating the dose of opium, that some persons are naturally ex-tremely susceptible, and others perhaps equally insusceptible to its effects; that, under the influence of habit, an individual becomes gradually less and less susceptible, and to an indefinite extent; and. lastly. that, in certain diseases, in which the cerebral centres are vastly depressed, and in others in which the whole energies of the nervous system are concentrated in some violent local affection, the dose required to produce a given effect is greatly augmented, and sometimes almost indefinitely. Among the former affections are delirium tremens, and the collapse at the commencement of certain fevers; among the latter, spasm of the stomach, severe colic, and, above all others, tetanus.

The commencing dose for an infant at birth, and for two or three weeks afterwards, should not exceed one-fortieth of a grain: and not more than from one-twentieth to one-tenth of a grain should be given to a child within the year. After this age, the dose may be regulated according to the rule of Dr. Young. (See pages 33-4.) The liquid preparations are generally preferable for infants.

Opium, in substance, may be taken in powder, pill, lozenge, or electuary. The form of pill is almost always preferred. For ordinary purposes, the pill should be made from powdered opium, and should not be kept very long before being used. But, when it is desirable, on any account, that it should operate very slowly, either an old pill, or one made directly from the plastic mass may be employed. The form of lozenge is used only when it is desirable that the medicine should be held in the mouth, and allowed slowly to dissolve; so as to act specially on the mouth and fauces, as may sometimes be desirable in coughs.

By the rectum, opium is used in the form of a suppository, or in that of an enema. The suppository may be made by rubbing the opium up with soap, or with cacao butter, and forming the mass into a cylindrical shape. The dose given by the rectum should not, at first, exceed twice the quantity administered by the mouth. A triple dose has not infrequently been given, and even more, without injury; but there is thought to be some risk from this larger quantity; at least, effects much greater than were anticipated or desired have sometimes been produced. The dose, then, in this way, at the first trial, may vary from one to four grains. In persons habituated to large doses of opium by the mouth, there might possibly be some danger from giving by the rectum twice the quantity usually taken in the former way. The susceptibility and absorbent power of the rectum may not bear the same relation to those of the stomach as before the habit was acquired. It would be safest, under these circumstances, to begin with a relatively very small dose, and increase, if necessary. For enema, one of the liquid preparations should generally be used, and administered in a wineglassful of pure water, or of some mucilaginous or amylaceous liquid, as flax-seed tea or solution of starch. The purposes for which opium is specially employed in this way, are, 1. to affect the system when the stomach will not retain the medicine; 2. to allay irritability of the stomach; 3. to check evacuations from the bowels; and 4. to relieve pain or other irritation in the rectum itself or neighbouring parts, as the genito-urinary apparatus. Thus strangury, spasm of the ureters, bladder, and urethra, dysmenorrhoea, priapism, chordee, etc., are more effectually and speedily relieved by opium, when given by the rectum, than by the mouth.

Opium, or one of its liquid preparations, is sometimes injected into the urethra or vagina, to relieve pain or irritation, or suppress discharges. It is also introduced into the cavity of a carious tooth to relieve toothache, and into the external meatus in earache. One of its liquid preparations is not unfrequently dropped into the eye, to allay irritability of the conjunctiva in ophthalmia. In the form of lotion, embrocation, cataplasm, or plaster, it is applied to various parts of the surface, to relieve pain, as in neuralgic affections, gouty and rheumatic pains or swellings, erysipelatous inflammation, various cutaneous eruptions, and irritated ulcers. But caution is always necessary, in these cases, not to use it so largely that, if absorbed, it might produce poisonous effects on the system; and special caution is necessary in infantile cases.