This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
On certain individuals opium produces peculiar effects, which differ according to their several idiosyncrasies. Thus, in some persons it causes much more than the usual degree of excitement, intoxicating them like alcohol, rendering them more or less delirious, and even producing convulsions. It is said to have these effects frequently on the negro and the Malay; hut I have not observed any special peculiarity of the kind in negroes of this region. In some, it gives rise, even in large doses, to headache, restlessness, and utter inability to sleep; while in others it acts with extraordinary energy as a soporific, even in small doses. There are individuals who, when they take it, always suffer excessively from nausea and vomiting, and occasionally with spasm of the stomach. Not a few, though they experience in the same manner as others its direct influence, yet are so much troubled with subsequent nausea, and general distress, as to preclude the use of the medicine in their cases. It is said even to purge some individuals, and in others to produce colic. I have repeatedly known it, in merely stimulant doses, to provoke an attack of neuralgic pain, in a person liable to this affection. The probability is, that some of these abnormal effects are owing to certain constituents of opium which are generally productive of no inconvenience, or to the peculiar mode in which the active principles may be combined or associated in the drug. It is certain that they may very frequently be obviated by particular modes of preparing the opium for use, consisting either in separating from it the obnoxious matters, or modifying the state of its active matter. Thus, morphia, or the black drop, or the aqueous extract, or the deodorized tincture, can be borne well, when opium itself, or other preparations would be rejected from the stomach, or, if retained, would occasion great inconvenience.
(!. Effects of Variation and Repetition of the Dose. As a general rule, in reference to the operation of opium connected with the quantity administered, the stimulant effect is protracted by a diminution, and shortened by an increase of the dose; while exactly the reverse is true of the subsequent narcotic effect, the intensity of which is proportionate to the amount taken.
There are few medicines to which the system becomes more rapidly habituated than this. By increasing the dose at short intervals, the quantity which may be swallowed, with present impunity, may be indefinitely augmented. An ounce or more is frequently consumed daily; and even pints of laudanum have been taken in the same time.