It has been pointed out that opium is a very variable body. Its chief action is dependent on morphine, which is found, as a rule, in the largest amounts. Occasionally the morphine strength is low, and thebaine or narcotine percentage high. In this event the tetanizing activities of these alkaloids becomes prominent. The alkaloids in opium seem to show a regular series of gradations in activity from morphine, through papaverine, codeine, and narcotine, to thebaine; in the former of which the cerebral activities are more manifest, while for narcotine and thebaine there is greater action on the spinal reflexes. Narcotine approaches strychnine in this respect.

Morphine is naturally much more certain in its action than opium. Papaverine is intermediate between morphine and codeine, but is a weak alkaloid. Opium possesses greater diaphoretic properties than morphine. Morphine produces more irritability of the bladder and occasions much greater itching of the skin than opium.

Codeine represents a middle activity. It is less depressing to the cerebral functions, and much less poisonous to the medulla; in children its stimulating action on the spinal cord may be manifest. It is much weaker than morphine, and rarely causes the unpleasant after-effects of this alkaloid. Codeine has a more selective action than morphine upon the nerves of the abdominal viscera; is less constipating than morphine or opium, and has comparatively little effect in medicinal doses upon the respiration.

The remaining alkaloids are present in very small quantities and are therapeutically negligible. Narceine was at one time considered highly poisonous, but later research has failed to confirm Claude Bernard's early dicta.