This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
I have already stated (p. 5) that the so-called action of a drug is not one-sided: it is the reaction between the drug and the organism. While drugs are circulating in the body they may modify the chemical nature and the physiological functions of various organs. In some cases the drug, after doing this, may again leave the organs and be eliminated without undergoing any essential change; but in other cases the chemical character of the drug itself undergoes an essential change during its sojourn in the body. Some organic substances undergo complete combustion, and are converted into carbonates, while others are converted into substances having a powerful physiological action, but perfectly different from that of the substance originally introduced into the body. These products of the decomposition of the drug may then, while circulating in the blood, or during the process of excretion, exert upon the organism a marked physiological action quite different from that of the original substance. Perhaps one of the most marked examples of this is to be found in morphine. Morphine lessens the irritability of nerve-centres, producing sleep, and having a marked sedative action upon the stomach in allaying vomiting, either when introduced directly into the stomach or injected into the circulation. This is its primary action; but in the body morphine undergoes certain ^ alterations and becomes partly converted into oxy-dimorphine, which appears to counteract the soporific action of morphine, and probably either oxy-dimorphine or some other product of the decomposition of morphine has an emetic action. The effect of these secondary products will manifest itself after the original dose of morphine has either been eliminated or undergone conversion into the products already mentioned; and thus the secondary action will be quite different from the primary, and instead of narcosis and quietness of the stomach, there will be excitement, and nausea or vomiting, which may require to be again counteracted by a larger dose of the original drug.
1 P. Ehrlich, 'Ueber die Methylenblaureaction der lebenden Nervensubstanz. Deutsche med. Wochenschrift, 1886, No. 4. Ibid. 1885.
It is evident that the relation between the primary and secondary effects of a drug will, if this explanation be correct, vary very much according to the relative solubility of the drug originally administered, and of the products of its decomposition. If the products of decomposition be more soluble, and more readily eliminated, than the drug itself, they will leave the organism before it, and their action will hardly appear; but if they are less soluble, and more slowly eliminated, their action may persist for a considerable length of time.