This section is from the book "Materia Medica: Pharmacology: Therapeutics Prescription Writing For Students and Practitioners", by Walter A. Bastedo. Also available from Amazon: Materia Medica: Pharmacology: Therapeutics: Prescription Writing for Students and Practitioners.
This, the methyl ester of morphine, is a weaker narcotic, and its power to allay pain and induce sleep is very much less than that of morphine. Yet where the lesser effect is sufficient, it has the following advantages over morphine: (1) It is not a vicious habit-drug; (2) it is not strongly constipating, and (3) it is less depressing to the respiration. Further, codeine differs from morphine in that it is excreted largely by the kidneys.
In Heinz's experiments with rabbits a dose of 1 1/2 grains (0.1 gm.) reduced the breathing from 92 to 60 in thirty-three minutes, but the individual inspirations were deeper, so that at the eightieth minute the air inspired had increased from 720 to 1000 c.c. per minute. With morphine, one-twentieth this amount reduced the rate of respiration and also the expired air to nearly one-half.
In allaying cough it is just as effective as morphine, but its dosage must be fully six times as large. A matter of note is that with a very slight increase beyond the hypnotic dose, a stimulating effect upon the cord may appear, with restlessness and increased reflex excitability instead of quiet and sleep. Its chief uses are to allay mild pain, especially abdominal pain, to promote sleep (usually with other hypnotics, such as trional or veronal), to quiet cough, and in diabetes. In a chronic disease like the last mentioned, and in tuberculous cough, codeine is just as useful and is preferred to morphine because of the ease with which a morphine-habit is established. The usual dose of codeine for cough is ¼ grain (0.015 gm.), and for pain, 1/2 grain (0.03 gm.), repeated every three or four hours; for hypodermatic use the phosphate is preferred because of its solubility. The author has seen two codeine habitues - they were broken of the habit without any trouble.
Apocodeine, an alkaloid prepared from codeine, has a different action. It is employed somewhat in the laboratory as a general paralyzant of sympathetic nerve-endings. In this respect it directly antagonizes epinephrine. In therapeutics it has been used slightly hypodermatically in dose of 1/2 grain (0.03 gm.) to promote intestinal peristalsis. It acts by cutting off splanchnic control of intestinal activity through the depression of the sympathetic nerve-endings, but is not a safe drug nor a very efficient one for the purpose.